Objective optical element and optical pickup device

ABSTRACT

Provided is an objective optical element which can appropriately correct degradation from spherical aberration upon fluctuation of a light source wavelength while maintaining light use efficiency, just by changing the magnification of the objective optical element, and which can record/reproduce information to/from different optical discs. Also provided is an optical pickup device using the objective optical element. When a light flux having two different wavelengths λ 11 , λ 12  (wherein λ 11 &lt;λ 12  and λ 12 −λ 11 =5 nm) within a range of wavelength λ 1  is introduced to the objective optical element to measure the wavefront aberration, the following third order and fifth order spherical aberrations in unit of λrms are obtained: SA3(λ 11 ), SA5(λ 11 ), SA3(λ 12 ), SA5(λ 12 ). If ΔSA3=|SA3(λ 12 )−SA3(λ 11 )|, ΔSA5=|SA5(λ 12 )−SA5(λ 11 )|, the following expression is satisfied: 0.18&gt;ΔSA3&gt;ΔSA5&gt;0 (1).

RELATED APPLICATIONS

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/JP2009/070887 filed Dec. 15, 2009.

This application claims the priority of Japanese application No.2008-320423 filed Jan. 17, 2008, the entire content of which is herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to an optical pickup device which canrecord and/or reproduce information compatibly for various types ofoptical discs, and to an objective lens for use in the same.

BACKGROUND ART

In recent years, research and development of a high density optical discsystem capable of recording and/or reproducing information (hereinafter,“record and/or reproduce” will be referred as “record/reproduce”) byusing a blue-violet semiconductor laser with a wavelength of about 400nm, are advancing rapidly. As an example, in the case of an optical discon which information is recorded and/or reproduced under thespecifications that NA is 0.85 and a light source wavelength is 405 nm,namely, in the case of the so-called Blu-ray Disc (hereinafter, BD), itis possible to record information of 25 GB per layer for an optical discwith a diameter of 12 cm, which is same in size as DVD (NA is 0.6,wavelength of a light source is 650 nm, and memory capacity is 4.7 GB).

In the meantime, a value as a product for an optical discplayer/recorder (optical information recording and reproducingapparatus) is not sufficient when the optical disc player/recorder onlycan record and/or reproduce information properly for the high densitydisc of this kind. In view of the realities that DVDs and CDs (CompactDiscs) on which various types of information are recorded are on themarket at present, only conducting information recording and/orinformation reproducing for the high density optical disc is notsufficient, and ability to conduct information recording and/orinformation reproducing properly also for DVDs and CDs owned by users,for example, enhances commercial value of the optical discplayer/recorder for the high density optical disc. With the aforesaidbackground, an optical pickup device to be built in the optical discplayer/recorder for a high density optical disc is requested to havecapability to conduct information recording and/or informationreproducing properly for any of high density optical discs, DVDs, andCDs, while maintaining compatibility.

As a method which enables to record and/or reproduce informationadequately for any of high density optical discs and DVDs and furtherfor CDs with maintaining compatibility, there can be considered a methodto selectively switch an optical system for high density optical discsand an optical system for DVDs and CDs, corresponding to the recordingdensity of an optical disc on which information will be recorded and/orreproduced. However, it is disadvantageous for the size-reduction andincreases the cost, because it requires a plurality of optical systems.

Accordingly, in order to simplify the structure of an optical pickupdevice and to intend the reduction of cost, it is preferable to make anoptical system for high density optical discs and an optical system forDVDs and CDs into a common optical system, and to reduce the number ofoptical parts constructing the optical pickup device as much aspossible, even in the optical pickup device with compatibility. Then,providing the common objective optical element which is arranged withfacing an optical disc, is most advantageous for simplification of theconstruction and cost reduction of the optical pickup device. Here, inorder to obtain an objective optical element which can be commonly usedfor plural kinds of optical discs for which differentrecording/reproducing wavelengths are used, it is required that a dillactive structure having a wavelength dependency in spherical aberration,is formed in the objective optical system, to reduce sphericalaberrations caused by a difference in wavelength and a difference inthickness of protective layers.

Patent Literature 1 discloses an objective optical element for recordingand/or reproducing information compatibly for high density optical discsand conventional DVDs and CDs.

CITATION LIST Patent Literature

Patent Literature 1: JP-B No. 4033239

SUMMARY OF INVENTION Technical Problem

Generally, wavelength of a light flux used for recording/reproducinginformation for a BD is 405 nm, which is shorter than wavelengths oflight fluxes used for recording/reproducing information for a DVD andCD, and NA of an objective lens for a BD is 0.85, which is a higher NAin comparison with the NA of 0.65 of an objective lens for a DVD.Accordingly, in an example that wavelength fluctuation is caused in alight source, spherical aberration caused when a BD is employed isgreater than that caused when a DVD is employed. A concrete example isprovided below. Under the assumption that they are compared using theratio of NA and wavelength simply, spherical aberrations areproportional to the fourth power of NA. Spherical aberration caused whena BD is employed is the about six times greater than that caused when aDVD is employed, where the about six times is given by(0.85/0.60)⁴*660/405, and that is required to be corrected by some kindof means.

In this situation, a diffractive structure can be employed to correctspherical aberration which is caused corresponding to a wavelengthfluctuation. However, in a common objective optical element forrealizing compatibility and for converging light fluxes on informationrecording surfaces of different optical discs, a diffractive structurefor realizing compatibility is arranged in a common area where bothlight fluxes are commonly used for recording and reproducinginformation, and specifications of the diffractive structure arenaturally fixed for achieving the compatibility. Therefore, sphericalaberration is hardly controlled corresponding to wavelength fluctuation,which is a problem. Especially, when the compatible objective opticalelement is made of a single lens in order to promote the reducing cost,a degree of the freedom of the design of a diffractive structure islimited more strictly in comparison with an objective optical elementcomposed of plural lenses and with an objective optical elementexclusively for a BD. Therefore, it can enlarge a possibility that thespherical aberration corresponding to wavelength fluctuation, which is aproblem.

To solve them, the inventor has focused on the way to change themagnification by moving, for example, a collimation lens arranged at aposition between a light source and an objective lens in the directionof the optical axis and to utilize the correction of sphericalaberration resulting from that. Such the collimation lens which ismovable in the direction of the optical axis as described above has beenequipped already as a standard component in many optical pickup deviceswhich can record/reproduce information for a multi-layer optical disc.By using the collimation lens for the correction, increase of excessivecost can be controlled, which is advantageous.

As for spherical aberrations, there are third-order sphericalaberration, fifth-order spherical aberration, and higher-order sphericalaberrations whose order is seventh or more. Among them, the third-orderspherical aberration and the fifth-order spherical aberration mainlyaffect the shape of a converged spot. When the magnification is changed,there is caused a trend that the third-order spherical aberration andthe fifth-order spherical aberration change in the same direction andthat their change amounts becomes smaller as the order becomes higher,as represented by the third-order spherical aberration>the fifth-orderspherical aberration>the seventh-order spherical aberration> . . . .Additionally, the change amount of the spherical aberrations whose orderis seventh or more is microscopic in comparison with the change amountsof the third-order and fifth-order spherical aberration. Therefore, itcan be said that the magnification can change only the third-orderspherical aberration and the fifth-order spherical aberration.

However, Patent Literature 1 does not disclose a problem to correct bothof the third-order spherical aberration and the fifth-order sphericalaberration. Further, it still does not disclose a way to use themagnification change for the correction. In the objective opticalelement in Patent Literature 1, there is used a high-order diffractivestructure. However, using a high-order diffractive structure causes aproblem that fluctuation in diffractive efficiency becomes large whenwavelength and temperature fluctuate, and an optical pickup devicebecomes difficult to handle. To explain the problem in concrete, whenthe form of the diffractive structure is designed to be optimized to them_(o)-th diffracted light at the wavelength λo, diffraction efficiencyη_(mo), of the m_(o)-th diffracted light generated when light withwavelength λ passes through a phase difference providing structure isrepresented by the following expression.η_(m0)=sin c ² [m ₀(λ₀/λ−1)]  [Math. 1]

As can be seen from FIG. 1 representing Math. 1 graphically, under thecondition that the reference wavelength is λo=405 nm and a light fluxwith wavelength, for example, of ±10 nm of the reference wavelengthenters the structure, deterioration in efficiency of the second-orderdiffracted light is about 1%. On the other hand, deterioration inefficiency of the fifth-order diffracted light is about 5% anddeterioration in efficiency of the eighth-order diffracted light is asmuch as about 13%. If the efficiency of the diffracted light isremarkably deteriorated, there is a possibility that information is notrecorded and/or reproduced for an optical disc adequately.

The present invention has been achieve in view of the above problems,and is aimed to provide an objective optical element and an opticalpickup device employing the same, where the objective optical elementcan preferably correct a deterioration of spherical aberration causedwhen the wavelength of a light source changes and can controlfluctuation in diffraction efficiency resulting from temperature changeto be small when the wavelength changes, to realize informationrecording/reproducing adequately for various optical discs.

Solution to Problem

An objective optical element described in item 1 is an objective opticalelement for use in an optical pickup device which comprises a firstlight source emitting a first light flux with a wavelength λ₁ (375nm≦λ₁≦435 nm), a second light flux emitting a second light flux with awavelength λ₂ (λ₁<λ₂), and an objective optical element, wherein theoptical pickup device records and/or produces information by convergingthe first light flux onto an information recording surface of a firstoptical disc including a protective layer with a thickness t1 andconverging the second light flux onto an information recording surfaceof a second optical disc including a protective layer with a thicknesst2 (t1<t2) to record and/or reproduce information, using the objectiveoptical element. The objective optical element is characterized in that

the objective optical element is a single lens and comprises a centralarea including an optical axis and a peripheral area arranged around thecentral area, wherein a central-area diffractive structure is arrangedin the central area,

the first light flux which has passed through the central area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the central area isconverged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced,

the first light flux which has passed through the peripheral area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the peripheral area isnot converged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced, and

the objective optical element satisfies the following expressions, whereSA3(λ₁₁), SA5(λ₁₁), SA3(λ₁₂), and SA5(λ₁₂) [unit: λrms] are third-orderspherical aberrations and fifth-order aberrations obtained when lightfluxes with two different wavelengths λ₁₁ and λ₁₂ being within the rangeof the wavelength λ₁ (where λ₁₁<λ₁₂ and λ₁₂−λ₁₁=5 nm) enter theobjective optical element and wavefront aberrations are measured:0.18>ΔSA3>ΔSA5>0  (1)

wherein ΔSA3=|SA3(λ₁₂)−SA3(λ₁₁)| and ΔSA5=|SA5(λ₁₂)−SA5(λ₁₁)|.

In a magnification correction, both of the third-order sphericalaberration (SA3) and the fifth-order spherical aberration (SA5)fluctuate so as to have the same polarity, and the absolute values ofthe fluctuation amounts hold SA3>SA5. Thereby, when 0.18>ΔSA3>ΔSA5>0 issatisfied, the third-order spherical aberration and the fifth-orderspherical aberration caused when the wavelength of the light sourcefluctuates can be reduced simultaneously by using the magnificationchange in one direction (in a direction to increase a convergent angleor increase a divergent angle). Especially, when the expression (1) issatisfied, the third-order spherical aberration and the fifth-orderspherical aberration can be excellently corrected only by themagnification change. Therefore, employing such the objective opticalelement makes the optical pickup device simplified. Herein, there are acase that both of the value of SA3(λ₁₂)−SA3(λ₁₁) and the value ofSA5(λ₁₂)−SA5(λ₁₁) become negative and a case that those become positive.Under the condition that the objective optical element is a plasticlens, when the both of the value of SA3(λ₁₂)−SA3(λ₁₁) and the value ofSA5(λ₁₂)−SA5(λ₁₁) are negative, a change amounts of sphericalaberrations caused when the temperature changes can be reduced, which ispreferable. On the other hand, when both of the value ofSA3(λ₁₂)−SA3(λ₁₁) and the value of SA5(λ₁₂)−SA5(λ₁₁) are positive, apossibility that a high-order diffractive structure is used can bereduced and a fluctuation in diffraction efficiency caused whenwavelength or temperature changes can be avoided from being enlarged,which is preferable.

Further, considering a combination of an objective optical element and acollimation lens that achieves a magnification of ×11 as a generalmagnification of an optical system in an optical pickup device for, forexample a BD, the change amount of the spherical aberrationcorresponding to the movement amount of the collimation lens is about0.17 λrms/mm for SA3 and is about 0.03 λrms/mm for SA5. Therefore, whenthe fluctuation amounts of the spherical aberrations corresponding towavelength change are kept within the expression (1), the movementamount of the collimation lens becomes about 1 mm, which does not harmdownsizing of the optical pickup device. Additionally, under theabove-described movement sensitivity of the collimation lens, it can becontrolled enough on the order of tens micrometers and does not need anaccurate drive control. Therefore, the cost reduction can be aimed.

An objective optical element described in item 2 is the objectiveoptical element of claim 1 characterized by satisfying the followingexpression:0.13>ΔSA3>0.03>ΔSA5>0  (1′).

Further, when both of expressions 0.13>ΔSA3>0.08 and 0.03>ΔSA5>0 aresatisfied, in the objective optical element which is a plastic lens,aberrations caused when the temperature changes can be reduced, which ispreferable. Especially, when the central-area diffractive structure isnot a structure composed of just one type of step structure which willbe described later but is a structure in which two types of structuressuch as blaze structures are overlapped together, it increases thedegree of freedom of designing so as to reduce aberrations caused whenthe temperature changes. Therefore, the both of the expressions0.13>ΔSA3>0.08 and 0.03>ΔSA5>0 are satisfied easily. Oh the other hand,when the central-area diffractive structure is composed of just one typeof single step structure, it is preferable that 0.09>ΔSA3>0.03>ΔSA5>0are satisfied.

An objective optical element described in claim 3 is the objectiveoptical element of item 1 characterized by satisfying the followingexpression:0.18>ΔSA3>0.06>ΔSA5>0  (1″).

An objective optical element described in item 4 is the objectiveoptical element of any one of items 1 to 3 characterized by satisfyingΔSA3:ΔSA5=α:1, wherein the value of α satisfies 4≦α≦9.

The present inventor has found after his earnest study that, in themagnification correction carried out by moving a collimation lens, theratio between changes of SA3 and SA5 is about 6:1. Accordingly, when theratio ΔSA3:ΔSA5=α:1 (4≦α≦9) is satisfied with the compatibility beingmaintained by optimizing the peripheral area, spherical aberrations canbe corrected excellently only by the magnification change even under thecondition that the wavelength fluctuates. Preferably, 5≦α≦9 issatisfied.

An objective optical element described in item 5 is the objectiveoptical element of any one of items 1 to 4, characterized in that

the objective optical element is used for the optical pickup devicefurther comprising a third light source emitting a third light flux witha wavelength. λ₃ (λ₂<λ₃), wherein the optical pickup device recordsand/or produces information by converging the third light flux onto aninformation recording surface of a third optical disc including aprotective layer with a thickness t3 (t2<t3) to record and/or reproduceinformation, using the objective optical element,

wherein the objective optical element further comprises an intermediatearea arranged between the central area and the peripheral area,

the first light flux which has passed through the central area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the central area isconverged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced,

the third light flux which has passed through the central area isconverged on the information recording surface of the third optical discso that information can be recorded and/or reproduced,

the first light flux which has passed through the intermediate area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the intermediate area isconverged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced,

the third light flux which has passed through the intermediate area isnot converged on the information recording surface of the third opticaldisc so that information can be recorded and/or reproduced,

the first light flux which has passed through the peripheral area isconverged on the information recording surface of the fast optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the peripheral area isnot converged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced, and

the third light flux which has passed through the peripheral area is notconverged on the information recording surface of the third optical discso that information can be recorded and/or reproduced.

When the objective optical element is used commonly forrecording/reproducing information for three different optical discs, thedesign of the objective optical element is further restricted and itsdegree of the freedom is reduced in order to achieve the compatibility,which furthermore enlarges the possibility that spherical aberrationsare deteriorated corresponding to the fluctuation in wavelength. Byapplying the present invention, spherical aberrations corresponding tothe fluctuation in wavelength can be corrected easily even in such theobjective optical element for achieving compatibility.

An objective optical element described in item 6 is the objectiveoptical element of any one of items 1 to 5, characterized by satisfyingthe following expressions, where W(λ₁₁) and W(λ₁₂) are wavefrontaberrations obtained when light fluxes with the wavelength λ₁₁ and thewavelength λ₁₂(λ₁₁<λ₁₂) enter the objective optical element andwavefront aberrations are measured:ΔW=W(λ₁₂)−W(λ₁₁)ΔW=C _(SAL)(20ρ⁶+6βρ⁴−6(3+β)ρ²+(4+β))+SAH  (2),where

W is a wavefront aberration (at a best focus) [λrms],

ρ is a relative pupil diameter (under an assumption that a value at acenter of an effective diameter is 0 and a value at a height of anoutermost position is 1),

C_(SAL) is a coefficient of low-order spherical aberrations,

SAH is spherical aberrations with seventh and more orders [λrms], and

β is an arbitral value within a range of 0≦β≦4.

In the expressions, SAH can be calculated by SAH=(SA7²+SA9²+SA11²+ . . .)^(1/2).

An objective optical element described in claim 7 is the objectiveoptical element of claim 6, characterized by satisfying the followingexpression:−0.030≦SAH≦0.030  (3).

When the value of SAH is kept in the range satisfying the expression(3), information can be recorded and/or reproduced properly, because itis sufficiently smaller than 0.070 λrms which is the so-called Marechalcriterion. Further, the spherical aberrations with seven and more orderscan be reduced and loss of the light amount can be reduced.

An objective optical element described in claim 8 is the objectiveoptical element of claim 6 or 7, characterized by satisfying thefollowing expression:0.00<C _(SAL)<0.03  (4).

Under the condition that the coefficient of low-order sphericalaberrations C_(SAL) is kept in the range satisfying the expression (4),when the magnification change is provided for correcting sphericalaberrations caused when the wavelength fluctuates, the resolution of themovement amount of; for example, the collimation lens can be enlarged tosome degree. Therefore, an accurate drive control is not required and acost reduction can be aimed. The following expression is more preferablysatisfied.0.01<C _(SAL)<0.03  (4′)

An objective optical element described in item 9 is the objectiveoptical element of any one of items 1 to 8, characterized in that adiffracted light flux with a diffraction order other than a zero-thorder has a maximum diffracted-light amount among diffracted lightfluxes generated when the first light flux enters the central-areadiffractive structure.

Because a diffracted light flux with a diffraction order other than azero-th order is used as a diffracted light flux generated when thefirst light flux enters the objective optical element, the objectiveoptical element of the present invention is different in shape andproperties from an objective optical element exclusive for the firstoptical disc. Accordingly, the possibility that the sphericalaberrations corresponding to the wavelength fluctuation is deterioratedincreases. By applying the present invention, spherical aberrationscorresponding to wavelength fluctuation can be corrected easily even insuch the objective optical element.

An objective optical element described in item 10 is the objectiveoptical element of any one of items 1 to 9, characterized by satisfyingthe following expression:0≦|N*d(n−1)/λ₁|≦50  (5),

where d is an average of step differences of ring-shaped zones [nm] of adiffractive structure arranged in the peripheral area,

n is a refractive index of a material of the objective optical elementat the wavelength λ1,

λ₁ is a wavelength [nm] of the first light flux, and

N is a number of the ring-shaped zones of the diffractive structurearranged in the peripheral area.

When the expression (5) is satisfied, the power of the diffraction is sosmall to less affect the diffraction efficiency and the ratioΔSA3:ΔSA5=α:1 (4≦α≦9) can easily be satisfied. Herein, in order tosatisfy 5≦α≦9, it is preferable to satisfy 0≦|N*d(n−1)/∥₁|≦25. When adiffractive structure is arranged in the peripheral area, it is morepreferable to satisfy 8≦|N*d(n−1)/≦18.

An objective optical element described in item 11 is the objectiveoptical element of item 10, characterized in that the peripheral area isa refractive surface, in the objective optical element.

An objective optical element described in item 12 is the objectiveoptical element of item 10, characterized in that the peripheral areacomprises a diffractive structure, in the objective optical element.

An optical pickup device described in item 13 is an optical pickupdevice comprising:

a first light source emitting a first light flux with a wavelength λ₁(375 nm≦λ₁≦435 nm);

a second light flux emitting a second light flux with a wavelength λ₂(λ₁<λ₂), and

an objective optical element,

wherein the optical pickup device records and/or produces information byconverging the first light flux onto an information recording surface ofa first optical disc including a protective layer with a thickness t1and converging the second light flux onto an information recordingsurface of a second optical disc including a protective layer with athickness t2 (t1<t2) to record and/or reproduce information, using theobjective optical element. The optical pickup device is characterized inthat

the objective optical element is a single lens and comprises a centralarea including an optical axis and a peripheral area arranged around thecentral area, wherein a central-area diffractive structure is arrangedin the central area,

the first light flux which has passed through the central area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the central area isconverged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced,

the first light flux which has passed through the peripheral area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the peripheral area isnot converged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced, and

the objective optical element satisfies the following expressions, whereSA3(λ₁₁), SA5(λ₁₁), SA3(λ₁₂), and SA5(λ₁₂) [unit: λrms] are third-orderspherical aberrations and fifth-order aberrations obtained when lightfluxes with two different wavelengths λ₁₁ and λ₁₂ being within the rangeof the wavelength λ₁ (where λ₁₁<₁₂ and λ₁₂−λ₁₁=5 nm) enter the objectiveoptical element and wavefront aberrations are measured:0.18>ΔSA3>ΔSA5>0  (1)

wherein ΔSA3=|SA3(λ₁₂)−SA3(λ₁₁)| and ΔSA5|SA5(λ₁₂)−SA5(λ₁₁)|.

An optical pickup device described in claim 14 is the optical pickupdevice of item 13, characterized by further comprising a third lightsource emitting a third light flux with a wavelength λ₃ (λ₂<λ₃), whereinthe optical pickup device records and/or produces information byconverging the third light flux onto an information recording surface ofa third optical disc including a protective layer with a thickness t3(t2<t3) to record and/or reproduce information, using the objectiveoptical element,

wherein the objective optical element further comprises an intermediatearea arranged between the central area and the peripheral area,

the first light flux which has passed through the central area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the central area isconverged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced,

the third light flux which has passed through the central area isconverged on the information recording surface of the third optical discso that information can be recorded and/or reproduced,

the first light flux which has passed through the intermediate area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the intermediate area isconverged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced,

the third light flux which has passed through the intermediate area isnot converged on the information recording surface of the third opticaldisc so that information can be recorded and/or reproduced,

the first light flux which has passed through the peripheral area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced,

the second light flux which has passed through the peripheral area isnot converged on the information recording surface of the second opticaldisc so that information can be recorded and/or reproduced, and

the third light flux which has passed through the peripheral area is notconverged on the information recording surface of the third optical discso that information can be recorded and/or reproduced.

An optical pickup device described in item 15 is the optical pickupdevice of item 13 or 14, characterized by further comprising amagnification changing means arranged at a position between the firstlight source and the objective optical element.

As the magnification changing means, there is preferably provided acombination of a coupling lens such as a collimation lens and a drivemeans for driving the coupling lens in the direction of the opticalaxis. The reason why is that a mechanism to move a coupling lens in thedirection of the optical axis in order to cope with a two-layer ormulti-layer optical disc has already been equipped as a standard memberin many optical pickup devices which can handle a two-layer ormultilayer optical discs, and the mechanism can be used also for themagnification changing means and can restrict cost increase of theoptical pickup device. As an example of a coupling lens, there ispreferably cited a collimation lens composed of a singe lens which iseasily controlled in terms of decentration adjustment and is easilymanufactured. Alternatively, there can be used a coupling lens composedof a single lens other than a collimation lens, a coupling lens composedof plural lenses, beam expander and a relay lens, as a coupling lens.Herein, when a collimation lens composed of a single lens is moved inthe direction of the optical axis and both of the value ofSA3(λ₁₂)−SA3(λ₁₁) and the value of SA5(λ₁₂)−SA5(λ₁₁) are negative, it ispreferable that the collimation lens is moved so as to be close to theobjective optical element when the wavelength is elongated. On the otherhand, when a collimation lens composed of a single lens is moved in thedirection of the optical axis and both of the value of SA3(λ₁₂)−SA3(λ₁₁)and the value of SA5(λ₁₂)−SA5(λ₁₁) are positive, it is preferable thatthe collimation lens is moved so as to be away from the objectiveoptical element when the wavelength is elongated. As another example,from a view point of reducing the movement amount of the lens, there canbe cited an embodiment that a positive lens and a negative lens areprovided as a coupling lens and only the positive lens is moved in thedirection of the optical axis, as a preferable embodiment.Alternatively, a liquid crystal device can be employed as themagnification changing means.

An optical pickup device relating to the present invention comprises atleast two light sources including a first light source and a secondlight source. However, it may further comprise a third light source. Theoptical pickup device relating to the present invention comprises alight-converging optical system for converging the first light flux onan information recording surface of the first optical disc and forconverging the second light flux on an information recording surface ofthe second optical disc. However, the light-converging optical systemmay be configured to converge the third light flux on an informationrecording surface of the third optical disc. The optical pickup devicerelating to the present invention comprises a light-receiving elementfor receiving light fluxes reflected on information recording surfacesof the first optical disc and the second optical disc. The opticalpickup device may further comprise a light-receiving element forreceiving light reflected on an information recording surface of thethird optical disc. In other words, the present invention can be appliedto an optical pickup device comprising two light sources and handlingtwo discs of the first optical disc and the second optical disc and toan objective optical element for use in the same, and the presentinvention can be applied also to an optical pickup device comprisingthree light sources and handling the third optical disc additionally tothe first optical disc and the second optical disc and to an objectiveoptical element for use in the same. It can be naturally applied to anoptical pickup device handling four or more discs and to an objectiveoptical element for use in the same.

The first optical disc has a protective substrate with a thickness of t1and an information recording surface. The second optical disc has aprotective substrate with a thickness of t2<t2) and an informationrecording surface. The third optical disc has a protective substratewith a thickness of t3 (t2<t3) and an information recording surface.Herein, it is preferable that the first optical disc is a BD (Blu-rayDisc), the second optical disc is a DVD, and the third optical disc is aCD. However, the discs are not limited to those. Each of the firstoptical disc, the second optical disc, and the third optical disc may bea multilayered optical disc including plural information recordinglayers. Herein, the thickness of the protective layer can have a valueof zero. If a protective film with a thickness of several to severaltens micrometers is applied on the optical disc, the thickness of theprotective layer includes the thickness of the protective film.

As for a BD, information is recorded and/or reproduced with an objectiveoptical element with NA of 0.85, and it has a protective layer with athickness about 0.1 mm. Further, a DVD represents a generic name ofoptical discs wherein information is recorded and/or reproduced with anobjective lens with NA in the range of about 0.60 to 0.67 and itsprotective layer has a thickness about 0.6 mm, and involves DVD-ROM,DVD-Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD+R and DVD+RW. In thepresent specification, a CD represents a generic name of optical discswherein information is recorded and/or reproduced by an objective lenswith NA in the range of about 0.45 to 0.53 and its protective layer hasthe thickness about 1.2 mm, and involves CD-ROM, CD-Audio, CD-Video,CD-R and CD-RW. As for a recording density, a BD has the highestrecording density, and recording densities of a DVD and CD decrease inthis order.

Thicknesses t1, t2, and t3 of the protective substrates preferablysatisfy the following conditional expressions (6), (7), and (8).However, the thicknesses are not limited to them.0.0750 mm≦t1≦0.1125 mm  (6)0.5 mm≦t2≦0.7 mm  (7)1.0 mm≦t3≦1.3 mm  (8)

The thickness of a protective substrate in this description means thethickness of a protective layer arranged on a surface of an opticaldisc. In other words, the thickness means the thickness of a protectivesubstrate extending from the surface of an optical disc to aninformation recording surface at the closest position to the surface.

In the present specification, each of the first light source, the secondlight source and the third light source is preferably a laser lightsource. Lasers such a semiconductor laser and a silicon laser arepreferably used for the laser light sources. The first wavelength λ₁ ofthe first light flux emitted from the first light source, the secondwavelength λ₂ (λ₂>λ₁) of the second light flux emitted from the secondlight source, and the third wavelength λ₃ (λ₃>λ₂) of the third lightflux emitted from the third light source are preferable to satisfy thefollowing conditional expressions (9) and (10).1.5×λ₁<λ₂<1.7×λ₁  (9)1.8×λ₁<λ₃<2.0×λ₁  (10)

When a BD, DVD, and CD are employed as the first optical disc, thesecond optical disc, and the third optical disc, respectively, thewavelength λ₁ of the first light source is preferably 375 nm or more,and 435 nm or less, and is more preferably 390 nm or more, and 420 nm orless; the second wavelength λ₂ of the second light source is preferably570 nm or more, and 680 nm or less, and is more preferably 630 nm ormore, and 680 nm or less; and the third wavelength λ₃ of the third lightsource is preferably 750 nm or more, and 850 nm or less, and is morepreferably 760 nm or more, and 820 nm or less.

Further, at least two light sources of the first light source, thesecond light source, and the third light source may be unitized. Theunitization means fixing and housing, for example, the first lightsource and the second light source into one package. However, it is notlimited to the above, the unitization in a broad sense involves asituation that two light sources are fixed so that aberration can not becorrected. Further, in addition to the light source, the light-receivingelement which will be described later, may also be provided as onepackage.

As the light-receiving element, a photodetector such as a photodiode ispreferably used. Light reflected on an information recording surface ofan optical disc enters into the light-receiving element, and signaloutputted from the light-receiving element is used for obtaining theread signal of information recorded in each optical disc. Further, achange in the light amount caused with a change in shape and a change inposition of a spot on the light-receiving element are detected toconduct a focus detection and a tracking detection. Based on thesedetections, the objective optical element can be moved for focusing andtracking operations. The light-receiving element may be composed of aplurality of photodetectors. The light-receiving element may also have amain photo detector and secondary photo detector. For example, thelight-receiving element is provided with a main photodetector whichreceives the main light used for recording and/or reproducinginformation, and two secondary photodetectors positioned on both sidesof the main photo detector, so as to receive secondary light fortracking adjustment by the two secondary photodetectors. Alternatively,the light-receiving element may comprise a plurality of light-receivingelements corresponding to respective light sources.

The light-converging optical system comprises an objective opticalelement. Preferably, the light-converging optical system furthercomprises a coupling lens such as a collimation lens, additionally tothe objective optical element. The coupling lens is arranged between theobjective optical element and the light sources, and means a single lensor a lens group which changes divergent angle of a light flux. Thecollimation lens is a kind of coupling lens and is a lens to convert alight flux which has entered the collimation lens into a parallel lightflux and to emit the resulting light. In the present specification, amagnification changing means can include a structure such that acoupling lens such as a collimation lens is moved in the direction ofthe optical axis, or a structure such that a light source is moved inthe direction of the optical axis.

Further, the light-converging optical system may also comprise anoptical element such as a diffractive optical element which divides alight flux emitted from a light source into a main light flux used forrecording and reproducing information and two secondary light fluxesused for operations such as a tracking operation. In the presentspecification, an objective optical element means an optical systemwhich is arranged to face an optical disc in an optical pickup deviceand has a function to converge a light flux emitted from a light sourceonto an information recording surface of the optical disc. The objectiveoptical element may be a glass lens, a plastic lens or a hybrid lens inwhich a diffractive structure formed of photo-curable resin is arrangedon a glass lens. The objective optical element preferably comprises arefractive surface which is an aspheric surface. Further, in theobjective optical element, it is preferable that a base surface (alsoreferred as a base aspheric surface) on which a diffractive structure isprovided is an aspheric surface. The objective optical element of thepresent invention is a single lens.

Further, when the objective optical element is a glass lens, a glassmaterial with glass transition point Tg of 500° C. or less is preferablyused. The glass transition point Tg of 480° C. or less is morepreferable. By using the glass material whose glass transition point Tgis 500° C. or less, the material can be molded at a comparatively lowtemperature. Therefore, the life of the mold can be prolonged.

Hereupon, a glass lens generally has larger specific gravity than aresin lens. Therefore, the objective optical element made of glass haslarger mass and applies a larger burden to an actuator which drives theobjective optical element. Therefore, when a glass lens is employed forthe objective optical element, a glass material having small specificgravity is preferably used for the objective optical element.Specifically, the specific gravity is preferably 3.0 or less, and ismore preferably 2.75 or less.

As examples of such a glass material, there can be cited Examples 1 to12 in JP-A No. 2005-306627. For example, Example 1 in JP-A No.2005-306627 exhibits grass transition point Tg of 460° C., the gravityof 2.58, refractive index nd of 1.594 and the Abbe number of 59.8.

Further, when a plastic lens is employed for the objective opticalelement, it is preferable that a resin material of cyclic olefins isused for the objective optical element. In the cyclic olefins, there ismore preferably used the resin material having: a refractive index atthe temperature 25° C. for wavelength 405 nm, which is within the rangeof 1.52 to 1.60; and a ratio of refractive index change dN/dT (° C.⁻¹)caused by a temperature change within the temperature range of −5° C. to70° C. for the wavelength 405 nm, which is within the range of −20×10⁻⁵to −5×10⁻⁵ (more preferably, −10×10⁻⁵ to −8×10⁻⁵). Further, when theobjective optical element employs a plastic lens, it is preferable thatthe coupling lens also employs a plastic lens.

The Abbe number of the material forming the objective optical element ispreferably 50 or more.

The objective optical element will be described below. At least oneoptical surface of the objective optical element comprises a centralarea and a peripheral area arranged around the central area. In the casethat the objective optical element is applied to an optical pickupdevice comprising the third light source additionally to the first lightsource and the second light source, at least one optical surface of theobjective optical element may comprise an intermediate area arrangedbetween the central area and the peripheral area. The central areapreferably is an area including the optical axis of the objectiveoptical element. Alternatively, a finite area including the optical axismay be provided as a unused area or a special-use area, and the centralarea may be provided around the finite area. The central area, theintermediate area and the peripheral area are preferably arranged on thesame optical surface. As shown in FIGS. 2 a and 2 b, it is preferablethat the central area CN, intermediate area MD, and peripheral area OTare provided on the same optical surface concentrically around theoptical axis as the center.

The central area of the objective optical element includes acentral-area diffractive structure. The central-area diffractivestructure may be composed of only one diffractive structure, or may becomposed of plural diffractive structures which are overlapped together.The term “overlapped” means literally that the structures aresuperimposed with their centers agreeing with the optical axis. Forexample, there can be cited an embodiment that a blaze structure and ablaze structure which will be described later are overlapped togetherand an embodiment that a step structure which will be described laterand a blaze structure are overlapped together.

If the objective optical element is applied to an optical pickup devicecomprising simply the first light source and the second light source,the peripheral area may be a refractive surface or an area on which anintermediate-area diffractive structure are arranged. If the objectiveoptical element is applied to an optical pickup device comprising thethird light source additionally to the first light source and the secondlight source, the intermediate area of the objective optical elementpreferably includes an intermediate-area diffractive structure. Theintermediate-area diffractive structure may be composed of only onediffractive structure, or may be composed of plural diffractivestructures which are overlapped together.

The peripheral area of the objective optical element may be a refractivesurface or an area on which a peripheral-area diffractive structure arearranged.

The area where the central-area diffractive structure is provided ispreferably 70% or more of the area of the central area on the objectiveoptical element. It is more preferably 90% or more of the area of thecentral area. The central-area diffractive structure is furthermorepreferably provided on the entire surface of the central area. The areawhere the intermediate-area diffractive structure is provided ispreferably 70% or more of the intermediate area on the objective opticalelement. It is more preferably 90% or more of the area of theintermediate area. The intermediate-area diffractive structure isfurther more preferably provided on the entire surface of theintermediate area. The area where the peripheral-area diffractivestructure is provided is preferably 70% or more of the peripheral areaon the objective optical element. It is more preferably 90% or more ofthe area of the peripheral area. The peripheral-area diffractivestructure is further more preferably provided on the entire surface ofthe peripheral area.

A diffractive structure used in the present specification, is a generalname of a structure which includes step differences and makes at least alight flux with a certain wavelength convergent or divergent bydiffractive action. For example, it involves a structure that is formedby plural unit forms which are arranged around the optical axis as theircenter (where the forms are also referred as ring-shaped structures) andthat is configured to converge light in a way that a light flux entersrespective unit forms and a wavefront of the light flux which has passedthrough the respective unit forms shifts by an almost integer multipleof wavelength or an integer multiple of wavelength at every neighboringring-shaped zones to form a new wavefront. The diffractive structurepreferably includes plural step differences. The step differences may bearranged along a direction perpendicular to the optical axis at periodicintervals, or may be arranged along a direction perpendicular to theoptical axis at non-periodic intervals. If a single aspheric lens, whichdoes not include an extra element such as a plate element additionallyto an objective optical element with a light-converging action, isemployed, incident angle of a light flux to the objective opticalelement depends on its height from the optical axis. Therefore, theamount of each step difference can slightly differ from the others.

It is preferable that the diffractive structure includes a plurality ofconcentric ring-shaped zones arranged around the optical axis as theircenter. Further, the diffractive structure can have various sectionalshapes (sectional shapes in a plane including the optical axis), and theshapes are divided broadly into a blaze structure and a step structure,according to their sectional shape including the optical axis.

The blaze structure is a structure that, as shown in FIGS. 3 a and 3 b,an optical element with a diffractive structure has a sectional shapewhich includes the optical axis and has a serrated shape. Thediffractive structure includes inclined surfaces which are notperpendicular to and are not parallel with a base aspheric surface. Inexamples shown in FIGS. 3 a to 3 d, it is assumed that the upper partpoints a side of the light source, the lower part points a side of theoptical disc, and the diffractive structure is formed on a plane as abase aspheric surface.

The step structure is a structure that, as shown in FIGS. 3 c and 3 d,an optical element with a diffractive structure has a sectional shapewhich includes the optical axis and has a plurality of small steppedbodies (which are referred as step units). In the present specification,“X-level” means that, in one step unit in the step structure,ring-shaped surfaces (which is sometimes referred as optical functionalsurfaces) corresponding to surfaces extending in the directionperpendicular to the optical axis (facing the direction perpendicular tothe optical axis) are divided with step differences to form a group ofthe X number of ring-shaped surfaces. Especially, a step structure ofthree or more levels includes small step differences and large stepdifferences.

A diffractive structure shown in FIG. 3 c is referred as a five-levelstep structure and a diffractive structure shown in FIG. 3 d is referredas a two-level step structure. The two-level step structure includes aplurality of concentric ring-shaped zones arranged around the opticalaxis as the center, and the plurality of ring-shaped zones have asectional shape which includes the optical axis of the objective opticalelement and is composed of plural step-difference surfaces Pa and Pbextending to be parallel with the optical axis, light-source-sideoptical functional surfaces Pc connecting light-source-side ends ofneighboring step-difference surfaces Pa and Pb, and optical-disc-sideoptical functional surfaces Pd connecting optical-disc-side ends ofneighboring step-difference surfaces Pa and Pb. Light-source-sideoptical functional surfaces Pc and optical-disc-side optical functionalsurfaces Pd are arranged alternately along the direction crossing theoptical axis.

In the step structure, a length of one step unit in a directionperpendicular to the optical axis is referred as pitch P.Step-difference surfaces preferably extend to be parallel with or almostparallel with the optical axis. Optical functional surfaces may beparallel with the base aspheric surface, or may be inclined with respectto the base aspheric surface.

The diffractive structure preferably is a structure in which a certainunit form is repeated periodically. Herein, “a certain unit form isrepeated periodically” naturally involves a form such that the same formis repeated on the same cycle. Further, “a certain unit form is repeatedperiodically” also involves a form such that unit forms eachcorresponding to one unit of the cycle are changed regularly such thattheir cycle is elongated gradually or shortened gradually.

When the diffractive structure has a blaze structure, it has a form thata serrated shape as a unit form is repeated. In the form, the sameserrated shape may be repeated as shown in FIG. 3 a, or a serrated shapebecomes greater or smaller in size gradually as a position in the formgoes further away from the optical axis. Alternatively, the form may beprovided by combining a form that a serrated shape becomes greater insize gradually and a form that a serrated shape becomes smaller in sizegradually. Herein, even in a form that a serrated shape changes in sizegradually, it is preferable that the amount of step differences in theoptical axis direction (or a direction where a passing ray travels) doesnot sufficiently change. Further, the form may have an area in which thestep differences of the blaze structure face the opposite direction tothe optical axis (the center), another area in which the stepdifferences of the blaze structure face the direction of the opticalaxis (the center), and a transition area which is arranged between thoseareas and is required to switch the orientation of the step differencesof the blaze structure. When an optical path difference provided by thediffractive structure is represented by an optical path differencefunction, the transition area corresponds to a point where the opticalpath difference function has an extreme value. When the optical pathdifference function has a point with an extreme value, the slope of theoptical path difference function becomes small. It increases the pitchesof the ring-shaped zones and controls the deterioration of transmittanceof the diffractive structure because of its shape error.

When the diffractive structure has a step structure, it can have a formthat, for example, the five-level step unit shown in FIG. 3 c isrepeated. Alternatively, the form may be a form that a step becomesgreater or smaller in size gradually as the position goes further awayfrom the optical axis. Herein, it is preferable that the amount of stepdifferences in the optical axis direction (or a direction where apassing ray travels) does not sufficiently change.

When there is provided an intermediate-area diffractive structure in theintermediate area of the objective optical element or a peripheral-areadiffractive structure in the peripheral area of the objective opticalelement, additionally to the central-area diffractive structure arrangedin the central area of the objective optical element, those structuresmay be formed on the different optical surfaces of the objective opticalelement, but it is preferable that those are arranged on the sameoptical surface. By forming them on the same optical surface, thedecentration error caused in a manufacturing process can be reduced,which is preferable. It is preferable that the central-area diffractivestructure and the intermediate-area diffractive structure orperipheral-area diffractive structure are arranged on thelight-source-side surface of the objective optical element rather thanthe optical-disc-side surface of the objective optical element.

The objective optical element converges each of the first light flux,the second light flux, and the third light flux each passing through thecentral area where the central-area diffractive structure is arranged,so as to form a converged spot. Preferably, the objective opticalelement converges the first light flux which passes through the centralarea where the central-area diffractive structure is arranged, onto aninformation recording surface of the first optical disc so thatinformation can be recorded and/or reproduced on the informationrecording surface of the first optical disc. It is preferable that theobjective optical element converges the second light flux which passesthrough the central area where the central-area diffractive structure isarranged, onto an information recording surface of the second opticaldisc so that information can be recorded and/or reproduced on theinformation recording surface of the second optical disc. It ispreferable that the objective optical element converges the third lightflux which passes through the central area where the central-areadiffractive structure is arranged, onto an information recording surfaceof the third optical disc so that information can be recorded and/orreproduced on the information recording surface of the third opticaldisc. Under the condition that the objective optical element handles thethird optical disc additionally to the first optical disc and the secondoptical disc, and that thickness t1 of the protective substrate of thefirst optical disc and thickness t2 of the protective substrate of thesecond optical disc are different in thickness from each other, it ispreferable that the central-area diffractive structure correctsspherical aberration caused because of the difference between thicknesst1 of the protective substrate of the first optical disc and thicknesst2 of the protective substrate of the second optical disc and/orspherical aberration caused because of the difference in wavelengthbetween the first light flux and the second light flux, for the firstlight flux and the second light flux each passing through thecentral-area diffractive structure. Further, it is preferable that thecentral-area diffractive structure corrects spherical aberration causedbecause of the difference between thickness t1 of the protectivesubstrate of the first optical disc and thickness t3 of the protectivesubstrate of the third optical disc and/or spherical aberration causedbecause of the difference in wavelength between the first light flux andthe third light flux, for the first light flux and the third light fluxeach passing through the central-area diffractive structure.

Under the condition that the intermediate-area diffractive structure isarranged on the objective optical element, the objective optical elementconverges each of the first light flux and the second light flux eachpassing through the intermediate area by using the structure so as toform a converged spot. Preferably, the objective optical elementconverges the first light flux which passes through the intermediatearea where the intermediate-area diffractive structure is arranged, ontoan information recording surface of the first optical disc so thatinformation can be recorded and/or reproduced on the informationrecording surface of the first optical disc. Under the condition thatthe intermediate-area diffractive structure is arranged on the objectiveoptical element, it is preferable that the objective optical elementconverges the second light flux which passes through the intermediatearea where the intermediate-area diffractive structure is arranged byusing the structure, onto an information recording surface of the secondoptical disc so that information can be recorded and/or reproduced onthe information recording surface of the second optical disc. It ispreferable that the intermediate-area diffractive structure correctschromatic spherical aberration caused because of the difference inwavelength between the first light flux and the second light flux.

As a preferable embodiment, there is cited an embodiment such that thethird light flux which has passed through the intermediate area is notused for recording and/or reproducing information for the third opticaldisc. It is preferable that the third light flux which has passedthrough the intermediate area does not contribute to forming a convergedspot on the information recording surface of the third optical disc. Inother words, under the condition that the intermediate-area diffractivestructure is arranged on the objective optical element, it is preferablethat the third light flux passing through the intermediate area formsflare light through the structure on the information recording surfaceof the third optical disc. As shown in FIG. 4, in a spot formed on theinformation recording surface of the third optical disc when the thirdlight flux passes through the objective optical element, there areprovided, in order from the optical-axis side (or from a central spotportion) toward the outside, central spot portion SCN whose light amountdensity is high, the intermediate spot portion SMD whose light amountdensity is lower than that of the central spot portion, and theperipheral spot portion SOT whose light amount density is higher thanthat of the intermediate spot portion and lower than that of the centralspot portion. The central spot portion is used for recording and/orreproducing information for the optical disc, and the intermediate spotportion and the peripheral spot portion are not used for recordingand/or reproducing information for the optical disc. In the abovedescription, this peripheral spot portion can be called flare light.However, also in the condition that the intermediate spot portion doesnot exist and the peripheral spot portion exists, around the centralspot portion, in other words, the condition that weak light forms alarge spot around the converged spot, the peripheral spot portion isreferred as flare light, too. In other words, the third light flux whichhas passed through the intermediate-area diffractive structure arrangedin the intermediate area of the objective optical element, forms theperipheral spot portion on the information recording surface of thethird light flux.

As a preferable embodiment that the peripheral area is provided, thereis cited an embodiment such that the first light flux which has passedthrough the peripheral area is used for recording and/or reproducinginformation for the first optical disc, and the second light flux andthe third light flux which have passed through the peripheral area arenot used for recording and/or reproducing information for the secondoptical disc and the third optical disc. It is preferable that thesecond light flux and the third light flux which have passed through theintermediate area do not contribute to forming a converged spot on theinformation recording surface of the second optical disc and the thirdoptical disc. In other words, under the condition that the objectiveoptical element includes the peripheral area, it is preferable that thesecond light flux and the third light flux passing through theperipheral area form flare light on the information recording surfacesof the second optical disc and the third optical disc. In other words,it is preferable that the second light flux and the third light fluxwhich have passed through the peripheral area of the objective opticalelement, form a peripheral spot portion on the information recordingsurface of each of the second optical disc and the third optical disc,respectively.

When the central-area diffractive structure is formed by overlappingplural diffractive structures with different actions together, thedirection of outgoing light can be changed for all the first light flux,the second light flux and the third light flux which have passed throughthe central-area diffractive structure. Therefore, even when all thefirst light flux, the second light flux, and the third light flux enterthe objective optical element at the same image-forming magnification(for example, as all the light fluxes enters it as parallel lightfluxes), aberrations generated because of using different types ofoptical discs can be corrected, which realizes compatibility.

The following description is a preferable condition required for theembodiment that, for example, the central-area diffractive structure isformed by overlapping a certain blaze structure (hereinafter, referredas the first basic structure) and anther blaze structure (hereinafter,the second basic structure) together. The first basic structure is ablaze structure as described above. The first basic structure makes theamount of X-th-order diffracted light of the first light flux which haspassed through the first basic structure larger than the amount ofdiffracted light of any other orders. The first basic structure makesthe amount of Y-th-order diffracted light of the second light flux whichhas passed through the first basic structure larger than the amount ofdiffracted light of any other orders. The first basic structure makesthe amount of Z-th-order diffracted light of the third light flux whichhas passed through the first basic structure larger than the amount ofdiffracted light of any other orders. Herein, the value of X ispreferably an odd integer. When the value of X is an odd number which isfive or less, the amount of step differences of the first basicstructure does not become excessively large. It makes its manufacturingprocess easy, controls the loss of light amount resulting from amanufacturing error, and reduces a fluctuation of diffraction efficiencycaused when wavelength changes, which is preferable.

In the first basic structure formed in the central area, its stepdifferences (surfaces parallel with the optical axis) preferably facethe direction opposite to the optical axis.

As described above, when the first basic structure in which thediffraction order for the first light flux is an odd number is arrangedwith its step differences facing the direction opposite to the opticalaxis, a working distance can be ensured sufficiently when a CD is used,even in a thick objective optical element whose thickness along theoptical axis as thick as it can be used for realizing compatibilitybetween three types of optical discs of BD, DVD, and CD, which ispreferable.

In a thick objective optical element whose thickness along the opticalaxis as thick as it can be used for realizing compatibility betweenthree types of optical discs of BD, DVD, and CD, it is preferable thatthe first basic structure has paraxial power for the first light flux,from the view point to ensure a working distance sufficiently when a CDis used. Herein, the term “having paraxial power” means that, when theoptical path difference function of the first basic structure isexpressed by Math. 3 which will be described below, the value of B2h2 isnot zero.

The second basic structure also is a blaze structure, as describedabove. The second basic structure makes the amount of L-th-orderdiffracted light of the first light flux which has passed through thesecond basic structure larger than the amount of diffracted light of anyother orders. The second basic structure makes the amount of M-th-orderdiffracted light of the second light flux which has passed through thesecond basic structure larger than the amount of diffracted light of anyother orders. The second basic structure makes the amount of N-th-orderdiffracted light of the third light flux which has passed through thesecond basic structure larger than the amount of diffracted light of anyother orders. Herein, the value of L is preferably an even integer. Whenthe value of L is an even number which is four or less, the amount ofstep differences of the second basic structure does not becomeexcessively large. It makes its manufacturing process easy, controls theloss of light amount resulting from a manufacturing error, and reduces afluctuation of diffraction efficiency caused when wavelength changes,which is preferable.

In the second basic structure formed in the central area, its stepdifferences (surfaces parallel with the optical axis) preferably facethe optical axis direction.

As described above, it is considered that the first basic structure inwhich the diffraction order for the first light flux is an odd number isarranged with its step differences facing the direction opposite to theoptical axis, the second basic structure in which the diffraction orderfor the first light flux is an even number is arranged with its stepdifferences facing the optical axis direction, and the first basicstructure and the second basic structure are overlapped together.Employing the above structure restricts that height of step differencesformed after they are overlapped together becomes excessively high, incomparison with the structure in which the first basic structure and thesecond basic structure are overlapped together with their stepdifferences facing the same direction. Corresponding to that, loss oflight amount resulting from a manufacturing error can be controlled anda fluctuation of diffraction efficiency when wavelength changes can becontrolled, which is preferable.

The structure does not simply realize the compatibility between threetypes of optical discs of BD, DVD, and CD, but also enables to providean objective optical element exhibiting well-balanced light useefficiency such that high light-use efficiency can be maintained foreach of the three types of optical discs of BD, DVD, and CD. Forexample, there can be provided an objective optical element exhibitingdiffraction efficiency of 80% or more for wavelength λ1, diffractionefficiency of 60% or more for wavelength λ2, and diffraction efficiencyof 50% or more for wavelength λ3. Further, there can be provided anobjective optical element exhibiting diffraction efficiency of 80% ormore for wavelength λ1, diffraction efficiency of 70% or more forwavelength λ2, and diffraction efficiency of 60% or more for wavelengthλ3. Furthermore, when step differences of the first basic structure facethe direction opposite to the optical axis, aberration caused when thewavelength fluctuates toward the longer-wavelength side can be changedin the under (under-corrected) direction. Thereby, aberrations generatedwhen the temperature of an optical pickup device increases can becontrolled. Under the condition that the objective lens is made ofplastic, an objective lens in which stable properties can be maintainedeven when the temperature changes can be provided.

In order that, under the condition that the objective lens is made ofplastic, a stable properties is maintained even when the temperaturechanges, it is preferable that both of the third-order sphericalaberration and the fifth-order spherical aberration caused in theobjective lens when the wavelength increases are under(under-corrected).

A more preferable central-area diffractive structure is a structureformed by overlapping the first basic structure wherein the values of|X|, |Y|, and |Z| are 1, 1, and 1, respectively, and the second basicstructure wherein the values of |L|, |M|, and |N| are 2, 1, and 1,respectively. By providing the above central-area diffractive structure,the height of step difference can be lowered. Accordingly, themanufacturing error can be more reduced, the loss of light amount can befurther more reduced, and the fluctuation of diffraction efficiencycaused when the wavelength changes can be controlled more preferably.

From the view point of the shape and the step difference amount of thecentral-area diffractive structure obtained after the first basicstructure and the second basic structure are overlapped together, thecentral-area diffractive structure formed by overlapping the first basicstructure wherein the values of |X|, |Y|, and |Z| are 1, 1, and 1,respectively, and the second basic structure wherein the values of |L|,|M|, and |N| are 2, 1, and 1, respectively, can be represented asfollows. It is preferable that the central-area diffractive structureincludes both of step differences facing the direction opposite to theoptical axis and step differences facing the optical axis direction, andthat step difference amount d11 of the step differences facing thedirection opposite to the optical axis and step difference amount d12 ofthe step differences facing the optical axis direction satisfy thefollowing conditional expressions. When the objective lens on which adiffractive structure is arranged is a single convex lens with anaspheric surface, the incident angle of a light flux to the objectivelens depends on its height from the optical axis. Therefore, even whenthe diffractive structure provides a uniform optical path difference,the step difference amount tends to be greater as the position of thestep difference is further away from the optical axis, generally. In thefollowing conditional expressions, the upper limit is obtained by beingmultiplied by 1.5. That is because such increase of the step differenceamount is considered. In the expressions, n represents a refractiveindex of the objective lens at the first wavelength λ1.0.6·(λ1/(n−1))<d11<1.5·(λ1/(n−1))0.6·(λ1/(n−1))<d12<1.5·(2λ1/(n−1))

Under the condition that, for example, the values of λ1 ranges from 390nm to 415 nm (from 0.390 μm to 0.415 μm) and the value of n ranges from1.54 to 1.60, the above conditional expressions can be represented asfollows.0.39 μm<d11<1.15 μm0.39 μm<d12<2.31 μm

Further, as a way to overlap the first basic structure and the secondbasic structure together, it is preferable to make the pitches the firstbasic structure and the second basic structure agree with each other,and to make the positions of all the step differences of the secondbasic structure and the step differences of the first basic structureagree with each other or to make the positions of all the stepdifferences of the first basic structure and the step differences of thesecond basic structure agree with each other.

When making the positions of all the step differences of the secondbasic structure and the step differences of the first basic structureagree with each other as describe above, the values of d11 and d12 ofthe central-area diffractive structure preferably satisfy the followingconditional expressions.0.6·(λ1/(n−1))<d11<1.5·(λ1/(n−1))0.6·(λ1/(n−1))<d12<1.5·(λ1/(n−1))

Under the condition that, for example, the values of λ1 ranges from 390nm to 415 nm (from 0.390 μm to 0.415 μm) and the value of n ranges from1.54 to 1.60, the above conditional expressions can be represented asfollows.0.39 μm<d11<1.15 μm0.39 μm<d12<1.15 μm

More preferably, they satisfy the following conditional expressions.0.9·(λ1/(n−1))<d11<1.5·(λ1/(n−1))0.9·(λ1/(n−1))<d12<1.5·(λ1/(n−1))

Under the condition that, for example, the values of λ1 ranges from 390nm to 415 nm (from 0.390 μm to 0.415 μm) and the value of n ranges from1.54 to 1.60, the above conditional expressions can be represented asfollows.0.59 μm<d11<1.15 μm0.59 μm<d12<1.15 μm

Further, when providing the first optical path difference providingstructure by overlapping the first basic structure wherein |X|, |Y|, and|Z| are 1, 1, and 1, respectively, and the second basic structurewherein |L|, |M|, and |N| are 2, 1, and 1, respectively, the first basicstructure can make the aberration caused when the wavelength becomesgreat to be under (under-corrected) (namely, makes wavelengthcharacteristics to be under), and inversely, the second basic structurecan make the aberration caused when the wavelength becomes great to beover (over-corrected) (namely, makes wavelength characteristics to beover). Therefore, the wavelength characteristic does not becomeexcessively under or excessively over, and the wavelengthcharacteristics being under at a suitable level can be obtained. Theterm “the wavelength characteristics being under at a suitable level”preferably corresponds to the condition that the absolute value ofaberration in terms of λrms is 150 or less. Thereby, even under thecondition that the objective lens is made of plastic, it is preferablefrom the view point that aberration change resulting from temperaturechange can be controlled to be small.

From the viewpoint to obtain “the wavelength characteristics being underat a suitable level” as described above, it is preferable that the firstbasic structure has a contributing rate which is more dominant than thatof the second basic structure. From the viewpoint to make thecontributing rate of the first basic structure more dominant than thatof the second basic structure, it is preferable that the average pitchof the first basic structure is smaller than the average pitch of thesecond basic structure. In other words, it can be represented thatpitches of step differences facing the direction opposite from theoptical axis are smaller than pitches of step differences facing thedirection of the optical axis, or it can be represented that, in thecentral-area diffractive structure, the number of step differencesfacing the direction opposite from the optical axis is greater than thenumber of step differences facing the direction of the optical axis.Furthermore, the average pitch of the first basic structure ispreferably a quarter or less of the average pitch of the second basicstructure, and is more preferably one sixth or less of the average pitchof the second basic structure. Providing the average pitch of the fastbasic structure which is a quarter or less (more preferably, one sixthor less) of the average pitch of the second basic structure ispreferable from the viewpoint to maintain a working distance for a CDadditionally to enable to obtain “the wavelength characteristics beingunder at a suitable level” as described above. In other words, it can besaid that the number of step differences facing the direction oppositefrom the optical axis is preferably four times or more as many as thenumber of step differences facing the direction of the optical axis, inthe central-area diffractive structure. It is more preferable that thenumber of step differences facing the direction opposite from theoptical axis is six times or more as many as that of the stepdifferences facing the direction of the optical axis.

The minimum pitch of the central-area diffractive structure ispreferably 15 μm or less, and is more preferably 10 μm or less. Theaverage pitch of the central-area diffractive structure is preferably 30μm or less, and is more preferably 20 μm or less. Providing such thestructure enables to obtain the “the wavelength characteristics beingunder at a suitable lever” as described above, and to separate a bestfocus position of necessary light which is used forrecording/reproducing information for the third optical disc and a bestfocus position of unnecessary light which is not used forrecording/reproducing information for the third optical disc away fromeach other to reduce a erroneous detection, where the necessary lightand the unnecessary light are generated from the third light flux whichhas passed through the central-area diffractive structure. Herein, anaverage pitch is obtained by calculating the total sum of pitches in thecentral-area diffractive structure and dividing the total sum by thenumber of step differences in the central-area diffractive structure.

It is preferable that, as for the third light flux which has passedthrough the central-area diffractive structure, the first-best focusposition where a spot formed by the third light flux has the strongestlight intensity and the second-best focus position where a spot formedby the third light flux has the second strongest light intensity satisfythe following expression. Herein, a best focus position indicates aposition where a beam waist becomes the minimum size within a certaindefocused range. The first-best focus position is a best focus positionof the necessary light which is used for recording/reproducinginformation for the third optical disc and the second-best focusposition is a best focus position of a light flux with the greatestlight amount out of the unwanted light which is not used forrecording/reproducing information for the third optical disc:0.05≦L/fl3≦0.35,

where fl3 [mm] is a focal length of the third light flux passing throughthe central-area diffractive structure and forming the first-best focus,and L [mm] is a distance between the first best focus and thesecond-best focus.

More preferably, the following conditional expression is satisfied.0.10≦L/fl3≦0.25

From the viewpoint that thin and elongated ring-shaped zones arepreferable in manufacturing process, the value of (“step differenceamount”/pitch) is preferably one or less, and more preferably 0.8 orless, for all the ring-shaped zones in the central-area diffractivestructure. Furthermore, the value of (“step difference amount”/pitch) ispreferably one or less, and more preferably 0.8 or less, for all thering-shaped zones in all the diffractive structures.

The image-side numerical aperture of the objective optical element,which is necessary for reproducing and/or recording information for thefirst optical disc, is defined as NA1. The image-side numerical apertureof the objective optical element, which is necessary for reproducingand/or recording information for the second optical disc, is defined asNA2 (NA1>NA2). The image side numerical aperture of the objectiveoptical element, which is necessary for reproducing and/or recordinginformation for the third optical disc, is defined as NA3 (NA2>NA3). NA1is preferably 0.6 or more, and 0.9 or less. It is more preferable thatNA1 is more preferably 0.85. NA2 is preferably 0.55 or more, and is 0.7or less. It is especially preferable that NA2 is 0.60 or 0.65. NA3 ispreferably 0.4 or more, and is 0.55 or less. It is especially preferablethat NA3 is 0.45 or 0.53.

It is preferable that the border of the central area and the peripheralarea in the objective optical element is formed in a portioncorresponding to the range being 0.9·NA3 or more and being 1.2·NA3 orless (more preferably, 0.95·NA3 or more, and 1.15·NA3 or less) under thecondition that the third light flux is used. More preferably, the borderof the central area and the peripheral area of the objective opticalelement is formed in a portion corresponding to NA3. Further, it ispreferable that the border of the peripheral area and the mostperipheral area of the objective optical element is formed in a portioncorresponding to the range being 0.9·NA2 or more, and being 1.2·NA2 orless (more preferably, being 0.95·NA2 or more, and being 1.15·NA2 orless) under the condition that the second light flux is used. Morepreferably, the border of the peripheral area and the most peripheralarea of the objective optical element is formed in a portioncorresponding to NA2.

When the third light flux which has passed through the objective opticalelement is converged on the information recording surface of the thirdoptical disc, it is preferable that spherical aberration has at leastone discontinuous portion. In that case, it is preferable that thediscontinuous portion exists in the range being 0.9·NA3 or more, andbeing 1.2·NA3 or less (more preferably, being 0.95·NA3 or more, andbeing 1.15·NA3 or less) under the condition that the third light flux isused.

Further, corresponding to the use of the optical pickup device,diffraction efficiencies of the central area for respective wavelengthscan be set properly. For example, in the case of the optical pickupdevice which records and reproduces information for the first opticaldisc, and which just reproduces information for the second and the thirdoptical discs, it is preferable that the diffraction efficiencies of thecentral area and/or the peripheral area are defined with consideringprimarily the diffraction efficiencies for the first light flux. On theother hand, in the case of the optical pickup device which onlyreproduces information for the first optical discs and which records andreproduces information for the second and third optical discs, it ispreferable that the diffraction efficiencies of the central area isdefined with considering primarily the diffraction efficiencies for thesecond and third light fluxes and the diffraction efficiencies of theperipheral area is defined with considering primarily the diffractionefficiency for the second light flux.

In any of the above cases, when the following conditional expression(11) is satisfied, the diffraction efficiency of the first light fluxcalculated by the area-weighted mean can be secured high.η11≦η21  (11)

In the expression, η11 expresses a diffraction efficiency of the firstlight flux in the central area, and η21 expresses a diffractionefficiency of the first light flux in the peripheral area. Hereupon,when the diffraction efficiencies of the central area are defined withconsidering primarily the light fluxes with the second and the thirdwavelengths, the diffraction efficiency of the first light flux of thecentral area is decreased. However, in the case where the numericalaperture of the first optical disc is larger than the numerical apertureof the third optical disc, when considered on the whole effectiveaperture of the first light flux, the decrease of diffraction efficiencyin the central area does not give so much large influence.

Hereupon, diffraction efficiency in the present specification can bedefined as follows.

(1) The transmittances of an objective optical element having the samefocal length, the same lens thickness, and the same numerical aperture,being formed of the same material, and excluding the central-area andperipheral-area diffractive structures, are measured for the centralarea and the peripheral area separately. In this case, the transmittanceof the central area is measured under the condition that the light fluxwhich enters the peripheral area is shielded, and the transmittance ofthe peripheral area is measured under the condition that the light fluxwhich enters the central area is shielded.(2) The transmittances of the objective optical element including thecentral-area and peripheral-area diffractive structures are measured forthe central area and the peripheral area separately.(3) The diffraction efficiencies of both areas are obtained by dividingthe results of (2) by the respective results of (1).

Further, it can be configured to make light use efficiency of two of thefirst through three light fluxes 65% or more, and to make a light useefficiency of the rest light flux 30% or more and 65% or less. In thiscase, the light flux exhibiting the light use efficiency being 30% ormore and 65% or less is preferably the third light flux.

Incidentally, light use efficiency mentioned here is calculated byLA/LB, where LA represents a light amount within an airy disc of aconverged spot formed on an information recording surface of an opticaldisc by the objective optical element on which the central-areadiffractive structure is formed (the intermediate-area diffractivestructure and the peripheral-area diffractive structure may be formedthereon), and LB represents a light amount within an airy disc of aconverged spot formed on an information recording surface of an opticalinformation recording medium by the objective optical element formed bythe same material, having the same focal length, the same thickness onthe axis, the same numerical aperture and same wavefront aberration, andexcluding the central-area diffractive structure, intermediate-areadiffractive structure and the peripheral-area diffractive structurethereon. Meanwhile, an airy disc mentioned here means a circle havingradius r′ whose center is on the optical axis of a converged spot. It isexpressed by r′=0.61·λ/NA.

It is preferable that the following conditional expression (12) issatisfied, where fl (mm) is a focal length of the objective opticalelement for the first light flux, and the central thickness d (mm) ofthe objective optical element:0.7≦d/fl≦1.5  (12)

It is more preferable that the following conditional expression (12′) issatisfied:1.0≦d/fl≦1.3  (12′)

Providing the above structure enables to maintain a working distance fora CD as the third optical disc without reducing the pitches of thediffractive structure and to make manufacturing of the objective opticalelement easy, and further enables to maintain high light use efficiency.

The first light flux, the second light flux, and the third light fluxmay enter the objective optical element as parallel light fluxes, or mayenter the objective optical element as divergent light fluxes orconvergent light fluxes. Preferably, the image-forming magnification m1of the objective optical element under the condition that the firstlight flux enters the objective optical element satisfies the followingexpression (13).−0.005≦m1≦0.005  (13)

When the second light flux enters the objective optical element as aparallel or almost parallel light flux, the image-forming magnificationm2 of the objective optical element under the condition that the secondlight flux enters the objective optical element, preferably satisfiesthe following expression (14).−0.005≦m2≦0.005  (14)

On the one hand, when the second light flux enters the objective opticalelement as a divergent light flux, the image-forming magnification m2 ofthe objective optical under the condition that the second light fluxenters the objective optical element, preferably satisfies theexpression (14′).−0.025<m2<0.00  (14′)

When the third light flux enters the objective optical element as aparallel or almost parallel light flux, it is preferable that theimage-forming magnification m3 of the objective optical element underthe condition that the third light flux enters the objective opticalelement, preferably satisfies the following expression (15). When thethird light flux is a parallel light flux, problems can be caused easilyin a tracking operation. However, the present invention can provideexcellent tracking characteristics even when the third light flux is aparallel light flux, and realizes recording and/or reproducing ofinformation for three different optical discs.−0.005≦m3≦0.005  (15)

On the one hand, when the third light flux enters the objective opticalelement as a divergent light flux, the image-forming manufacture m3 ofthe objective optical element under the condition that the third lightflux enters the objective optical element, preferably satisfies theexpression (15′).−0.025<m3≦0.00  (15′)

The working distance (WD) of the objective optical element when thethird optical disc is used is preferably 0.15 mm or more, and 1.5 mm orless. It is more preferably 0.3 mm or more, and 1.20 mm or less. Next,the WD of the objective optical element when the second optical disc isused is preferably 0.2 mm or more, and 1.3 mm or less. Furthermore, theWD of the objective optical element when the first optical disc is usedis preferably 0.25 mm or more, and 1.0 mm or less.

The optical information recording and reproducing apparatus according tothe present invention includes an optical disc drive apparatus includingthe above described optical pickup device.

Herein, the optical disc drive apparatus installed in the opticalinformation recording and reproducing apparatus will be described. Thereis provided an optical disc drive apparatus employing a system of takingonly a tray which can hold an optical disc under the condition that theoptical disc is mounted thereon, outside from the main body of theoptical information recording and reproducing apparatus in which opticalpickup device is housed; and a system of taking out the main body of theoptical disc drive apparatus in which the optical pickup device ishoused.

The optical information recording and reproducing apparatus using eachof the above described systems, is generally provided with the followingcomponent members but the members are not limited to them: an opticalpickup device housed in a housing; a drive source of the optical pickupdevice such as seek-motor by which the optical pickup device is movedtoward the inner periphery or outer periphery of the optical disc foreach housing; traveling means having a guide rail for guiding theoptical pickup device toward the inner periphery or outer periphery ofthe optical disc; and a spindle motor for rotation driving of theoptical disc.

The optical information recording and reproducing apparatus employingthe former system is preferably provide with, other than these componentmembers, a tray which can hold the optical disc with the optical discbeing mounted thereon, and a loading mechanism for slidably moving thetray. The optical information recording and reproducing apparatusemploying the latter system preferably does not include the tray andloading mechanism, and it is preferable that each component member isprovided in the drawer corresponding to chassis which can be taken outoutside.

Advantageous Effect of Invention

According to the present invention, there can be provided an objectiveoptical element and an optical pickup device, where the objectiveoptical element allows recording and/or reproducing information properlyfor various kinds of discs, by maintaining suitable light use efficiencyand controlling deterioration of spherical aberration caused whenwavelength of a light flux fluctuates.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of deterioration in diffractionefficiency when wavelength fluctuation causes.

FIG. 2 a is a diagram of an example of objective optical element OLrelating to the invention which is viewed in the optical axis direction,and FIG. 2 b is its sectional view.

FIGS. 3 a to 3 d schematically show sectional views of some examples ofdiffractive structures to be provided on the objective optical elementOL relating to the invention.

FIG. 4 is a diagram showing a form of a spot formed by the objectiveoptical element relating to the invention.

FIG. 5 is a diagram schematically showing the structure of an opticalpickup device relating to the invention.

FIGS. 6 a to 6 d show sectional views of exemplified diffractivestructures employed in Examples.

DESCRIPTION OF EMBODIMENTS

Referring to the drawings, an embodiment of the present invention willbe described below. FIG. 5 is a diagram schematically showing aconstruction of the optical pickup device PU1 of the present embodimentcapable of recording and/or reproducing information adequately for a BD,DVD and CD which are different optical discs. The optical pickup devicePU1 can be mounted in the optical information recording and reproducingapparatus. Herein, the first optical disc is a BD, the second opticaldisc is a DVD, and the third optical disc is a CD. Hereupon, the presentinvention is not limited to the present embodiment.

The optical pickup device PU1 comprises objective optical element OL,quarter wavelength plate QWP, collimation lens COL, polarization beamsplitter BS, dichroic prism DP, first semiconductor laser LD1 (the firstlight source) which emits a laser light flux with wavelength of λ₁=405nm (the first light flux) when recording/reproducing information for aBD, and laser unit LDP provided by unitizing second semiconductor laserLD2 (the second light source) which emits a laser light flux withwavelength of λ₂=660 nm (the second light flux) when recording and/orreproducing information for a DVD and third semiconductor laser LD3 (thethird light source) emitting a laser light flux with wavelength ofλ₃=785 nm (the third light flux) when recording and/or reproducinginformation for a CD. The optical pickup device PU1 further comprisessensor lens SEN and light-receiving element PD as a photodetector.

As shown in FIGS. 2 a and 2 b, in objective optical element OL being asingle lens of the present embodiment, there are formed central area CNincluding the optical axis, intermediate area MD arranged around thecentral area, and peripheral area OT further arranged around theintermediate area which are formed concentrically around the opticalaxis as their center. A central-area diffractive structure is formed incentral area CN and an intermediate-area diffractive structure is formedin intermediate area MD, which are not illustrated in the figures.Further, peripheral area OT is provided as an area on which adiffractive structure is formed or an area which does not include adiffractive structure and is composed of a refractive surface.

Assuming that SA3(λ₁₁), SA5(λ₁₁), SA3(λ₁₂), and SA5(λ₁₂) are third-orderspherical aberrations and fifth-order aberrations obtained when lightfluxes with two different wavelengths λ₁₁ and λ₁₂ being within the rangefrom 375 nm to 435 nm (where λ₁₁<λ₁₂ and λ₁₂−λ₁₁=5 nm) enter objectiveoptical element OL and wavefront aberrations are measured in units ofλrms, the following conditional expressions (1) and (5) are satisfied.0.18>ΔSA3>ΔSA5>0  (1)0≦|N*d*(n−1)/λ₁|≦50  (5),

In the expressions,ΔSA3=|SA3(λ₁₂)−SA3(λ₁₁)|,ΔSA5=|SA5(λ₁₂)−SA5(λ₁₁)|,

d is an average step difference [nm] of ring-shaped zones of theperipheral-area diffractive structure,

n is a refractive index of a material of the objective optical element,

λ₁ is a wavelength [nm] of the first light flux, and

N is a number of the ring-shaped zones of the peripheral-areadiffractive structure.

It is more preferable that the expression ΔSA3:ΔSA5=α:1 (where 4≦α≦9) issatisfied.

Assuming that W(λ₁₁) and W(λ₁₂) are wavefront aberrations obtained whenlight fluxes with wavelength λ₁₁ and wavelength λ₁₂ (λ₁₁<λ₁₂) enter theobjective optical element and wavefront aberrations are measured, thefollowing expressions are preferably satisfied.ΔW=W(λ₁₂)−W(λ₁₁)ΔW=C _(SAL)(20ρ⁶+6βρ⁴−6(3+β)ρ²+(4+β))+SAH  (2)

In the expressions, W is a wavefront aberration (at the best focus)[λrms],

ρ is a relative pupil diameter (under an assumption that a value at acenter of an effective diameter is zero and a value at a height of anoutermost position is one),

C_(SAL) is a coefficient of low-order spherical aberrations,

SAH is spherical aberrations with seventh or more orders [λrms], and

β is an arbitral value within a range of 0≦β≦4.

A divergent light flux as the first light flux (λ₁=405 nm) emitted fromblue-violet semiconductor laser diode LD1, as illustrated by solidlines, passes through dichroic prism DP and passes through polarizationbeam splitter BS. After that, the light flux passes through collimationlens COL and becomes a parallel light flux. The parallel light flux isconverted from linear polarized light into circular polarized light byquarter wavelength plate QWP. The diameter of the converted light fluxis regulated by a stop which is not illustrated, and the resulting lightflux enters objective optical element OL. The light flux which isconverged by the central area, the intermediate area, and the peripheralarea, is formed into a spot on information recording surface RL1 of a BDthrough protective substrate PL1 with the thickness of 0.1 mm.

The reflection light flux which is modulated on the informationrecording surface RL1 by information pits passes through objectiveoptical element OL and the stop which is not illustrated again. Afterthat, the light flux is converted from circular polarized light intolinear polarized light by quarter wavelength plate QWP. Then,collimation lens COL converts the light flux into a convergent lightflux. The convergent light flux is reflected by polarization beamsplitter BS and is converged through sensor lens SEN on the lightreceiving surface of the light-receiving element PD. Then, informationrecorded in a BD can be read based on the output signal oflight-receiving element PD, by focusing or tracking objective opticalelement OL using two-axis actuator AC1. Herein, when wavelengthfluctuation is caused in the first light flux, spherical aberrationgenerated because of that can be corrected by changing the position ofcollimation lens COL as a magnification changing means in the directionof the optical axis so as to change a divergent angle or convergentangle of a light flux entering the objective optical element. When a BDincludes plural information recording surfaces, aberrations generatedcorresponding to the difference in thickness of protective substrates ofthe information recording surfaces may be corrected by changing theposition of collimation lens COL in the direction of the optical axis soas to change a divergent angle or convergent angle of a light fluxentering the objective optical element.

A divergent light flux as the second light flux (λ₂=660 nm) emitted fromsemiconductor laser LD2 of laser unit LDP, as illustrated by dottedlines, is reflected by dichroic prism DP and passes through polarizationbeam splitter BS and collimation lens COL. After that, the light flux isconverted from circular polarized light into linear polarized light byquarter wavelength plate QWP. The resulting light flux enters intoobjective optical element OL. Herein, the light flux converged by thecentral area and the intermediate area of the objective optical elementOL (the light flux passing through the peripheral area is made intoflare light, and forms the peripheral spot portion), is formed into thecentral spot portion on information recording surface RL2 of a DVDthrough the protective substrate PL2 with a thickness of 0.6 mm.

The reflection light flux which is modulated on the informationrecording surface RL2 by information pits passes through objectiveoptical element OL again. After that, the light flux is converted fromcircular polarized light into linear polarized light by quarterwavelength plate QWP. Then, collimation lens COL converts the light fluxinto a convergent light flux. The convergent light flux is reflected bypolarization beam splitter BS and is converged through sensor lens SENon the light receiving surface of light-receiving element PD. Then,information recorded in a DVD can be read based on the output signal oflight-receiving element PD.

A divergent light flux as the third light flux (λ₃=785 nm) emitted fromsemiconductor laser LD3 of laser unit LDP, as illustrated bydashed-and-dotted lines, is reflected by dichroic prism DP and passesthrough polarization beam splitter BS and collimation lens COL. Afterthat, the light flux is converted from circular polarized light intolinear polarized light by quarter wavelength plate QWP. The resultinglight flux enters into objective optical element OL. Herein, the lightflux converged by the central area of the objective optical element OL(the light flux passing through the intermediate area and the peripheralarea is made into flare light, and forms the peripheral spot portion),is formed into the central spot portion on information recording surfaceRL3 of a CD through the protective substrate PL3 with a thickness of 1.2mm.

The reflection light flux which is modulated on the informationrecording surface RL3 by information pits passes through objectiveoptical element OL again. After that, the light flux is converted fromcircular polarized light into linear polarized light by quarterwavelength plate QWP. Then, collimation lens COL converts the light fluxinto a convergent light flux. The convergent light flux is reflected bypolarization beam splitter BS and is converged through sensor lens SENon the light receiving surface of light-receiving element PD. Then,information recorded in a CD can be read based on the output signal oflight-receiving element PD.

EXAMPLES

Hereinafter, Examples which can be used for the aforesaid embodimentwill be explained as follows. In the followings (including lens data intables), the power of 10 will be expressed as by using “E” (for example,2.5×10⁻³ will be expressed as 2.5E-3). The optical surfaces of theobjective optical element are respectively formed into aspheric surfaceswhich are regulated by an expression obtained by substitutingcoefficients shown in the tables to the expression of Math 2.

$\begin{matrix}{{X(h)} = {\frac{\left( {h^{2}\text{/}r} \right)}{1 + \overset{\_}{{)1} - {\left( {1 + \kappa} \right)\left( {h\text{/}r} \right)^{2}}}} + {\sum\limits_{i = 0}^{10}{A_{2i}h^{2i}}}}} & \left\lbrack {{Math}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Herein, X(h) represents the axis along the optical axis (the directionof traveling light is defined as a positive direction), κ is a conicconstant, A_(i) is an aspheric surface coefficient, h is the height fromthe optical axis, and r is the paraxial curvature radius.

In Examples using a diffractive structure, an optical path differenceprovided by the di active structure for the light flux with eachwavelength is defined by an expression obtained by substituting thecoefficients shown in the tables into the optical path differencefunction represented by Math 3.Φ=mλΣB _(2i) h ^(2i)   [Math. 3](Unit:mm)

In the expression, m is the number of diffraction order, λ is awavelength of an incident light flux, B₂, is a coefficient of theoptical path difference function, and his a height from the opticalaxis.

FIGS. 6 a to 6 d show sectional shapes of exemplified diffractivestructures employed in Examples. FIG. 6 a shows a seven-level stepstructure as a 3λ-area structure (1/−2/−3) of Example 1 which will beshown later. FIG. 6 b shows a five-level step structure as a 3λ-areastructure (1/−1/−2) of Example 2 which will be shown later. FIG. 6 cshows a two-level step structure as a 3λ-area structure (0/0/1) ofExample 3 which will be shown later. FIG. 6 d shows a three-level stepstructure as a 2λ-area structure (0/−1/*) of Example 3 which will beshown later.

In all the Examples, the following expressions hold, where λ₁₂ is 410 nmand λ₁₁ is 405 nm.ΔSA3=|SA3(λ₁₂)−SA3(λ₁₁)|ΔSA5=|SA5(λ₁₂)−SA5(λ₁₁)|

In Examples 1 to 4, both of the value of SA3(λ₁₂)−SA3(λ₁₁) and the valueof SA5(λ₁₂)−SA5(λ₁₁) are positive. On the other hand, in Example 5, bothof the value of SA3(λ₁₂)−SA3(λ₁₁) and the value of SA5(λ₁₂)−SA5(λ₁₁) arenegative.

Example 1

Table 1 shows lens data of Example 1. In Example 1, the central-areadiffractive structure has a seven-level structure shown in FIG. 6 a. Afirst-order diffracted light flux has a maximum diffraction-light amountamong diffracted light fluxes generated when the first light flux entersthe central-area diffractive structure, a minus-second-order diffractedlight flux has a maximum diffraction-light amount among diffracted lightfluxes generated when the second light flux enters the central-areadiffractive structure, and a minus-third-order diffracted light flux hasa maximum diffraction-light amount among diffracted light fluxesgenerated when the third light flux enters the central-area diffractivestructure. The intermediate-area diffractive structure has a three-levelstep structure shown in FIG. 6 d. A zeroth-order diffracted light fluxhas a maximum diffraction-light amount among diffracted light fluxesgenerated when the first light flux enters the intermediate-areadiffractive structure, and a minus-first-order diffracted light flux hasa maximum diffraction-light amount among diffracted light fluxesgenerated when the second light flux enters the second basic structure.The peripheral-area diffractive structure may has any one of a blazestructure and a step structure, and is a structure that a second-orderdiffracted light flux has a maximum diffraction-light amount amongdiffracted light fluxes generated when the first light flux enters theperipheral-area diffractive structure.

In Example 1, ΔSA3 is 0.060 λrms and ΔSA5 is 0.002 λrms, which satisfythe above described expression (1). The material of the objectiveoptical element is plastic. The value of |N*d*(n−1)/λ₁| is 8 and itsatisfies the above described expression (5).

TABLE 1 (Example 1) Focal length of the f₁ = 2.20 mm f₂ = 2.36 mm f₃ =2.44 mm objective lens Numerical aperture NA1: 0.85 NA2: 0.60 NA3: 0.47Magnification m1: 0 m2: −1/189 m3: −1/36 i-th di ni di ni di ni surfaceri (405 nm) (405 nm) (660 nm) (660 nm) (785 nm) (785 nm) 0 ∞ 450.0089.00 1 0.0 0.0 0.0 (Stop (φ3.74 mm) (φ2.86 mm) (φ2.37 mm) diameter) 2-11.4760 2.670 1.5592 2.670 1.5397 2.670 1.5363 2-2 1.4580 2-3 1.5149 3−2.7553 0.694 0.587 0.376 4 ∞ 0.0875 1.6196 0.600 1.5773 1.200 1.5709 5∞ Surface No. 2-1 2-2 2-3 3 Area h ≦ 1.190 1.190 ≦ h ≦ 1.434 1.434 ≦ h ≦1.87 — Aspheric κ −7.0931E−01 −6.3289E−01 −6.2123E−01 −4.6840E+01surface A0 0.0000E+00 −7.0823E−03 4.0539E−03 0.0000E+00 coefficient A49.1173E−03 8.4553E−03 1.1945E−02 8.8950E−02 A6 8.5249E−03 −1.7568E−03−1.1383E−04 −9.2978E−02 A8 −1.5985E−02 2.7992E−03 2.8043E−03 7.7776E−02A10 1.8088E−02 −1.2008E−03 −1.5393E−03 −4.4989E−02 A12 −8.9726E−032.4437E−04 2.2720E−04 1.4056E−02 A14 1.0780E−03 2.2226E−04 2.3820E−04−1.7533E−03 A16 6.7692E−05 −2.0287E−04 −1.6602E−04 −9.5890E−06 A184.1562E−04 5.9236E−05 4.5324E−05 0.0000E+00 A20 −1.6181E−04 −5.2971E−06−4.6289E−06 0.0000E+00 Optical Diffraction 1/−2/−3 0/−1/* 2/*/* pathorder m difference B2 −8.2634E+00 −9.5340E−+00 1.2526E−02 function B45.8703E−01 −4.1041E−00 6.3119E−02 B6 −3.1806E−01 −2.0713E+00 1.0011E−03B8 1.4808E−01 1.8892E+00 −5.0437E−03 B10 −2.9436E−02 −3.4836E−01−5.7434E−03 Wavelength characteristic: +5 nm ΔSA3: 0.060 ΔSA5: 0.002|d(n − 1)/λ₁ * N| = 8

Example 2

Table 2 shows lens data of Example 2. In Example 2, the central-areadiffractive structure has a five-level structure shown in FIG. 6 b. Afirst-order diffracted light flux has a maximum diffraction-light amountamong diffracted light fluxes generated when the first light flux entersthe central-area diffractive structure, a minus-first-order diffractedlight flux has a maximum diffraction-light amount among diffracted lightfluxes generated when the second light flux enters the central-areadiffractive structure, and a minus-second-order diffracted light fluxhas a maximum diffraction-light amount among diffracted light fluxesgenerated when the third light flux enters the central-area diffractivestructure. The intermediate-area diffractive structure has a three-levelstep structure shown in FIG. 6 d. A zeroth-order diffracted light fluxhas a maximum diffraction-light amount among diffracted light fluxesgenerated when the first light flux enters the intermediate-areadiffractive structure, and a minus-first-order diffracted light flux hasa maximum diffraction-light amount among diffracted light fluxesgenerated when the second light flux enters the second basic structure.The peripheral-area diffractive structure may has any one of a blazestructure and a step structure, and is a structure that a fourth-orderdiffracted light flux has a maximum diffraction-light amount amongdiffracted light fluxes generated when the first light flux enters theperipheral-area diffractive structure.

In Example 2, ΔSA3 is 0.062 λrms and ΔSA5 is 0.013 λrms, which satisfythe above described expression (1). The material of the objectiveoptical element is plastic. The value of |N*d*(n−1)/λ₁| is 12 and itsatisfies the above described expression (5).

TABLE 2 (Example 2) Focal length of the f₁ = 2.20 mm f₂ = 2.35 mm f₃ =2.45 mm objective lens Numerical aperture NA1: 0.85 NA2: 0.61 NA3: 0.46Magnification m1: 0 m2: −1/63 m3: −1/89 i-th di ni di ni di ni surfaceri (405 nm) (405 nm) (660 nm) (660 nm) (785 nm) (785 nm) 0 ∞ 150.0 220.01 0.0 0.0 0.0 (Stop (φ3.74 mm) (φ2.87 mm) (φ2.30 mm) diameter) 2-11.4848 2.680 1.5592 2.680 1.5397 2.680 1.5363 2-2 1.4633 2-3 1.5024 3−2.7532 0.685 0.585 0.395 4 ∞ 0.0875 1.6196 0.600 1.5773 1.100 1.5709 5∞ Surface No. 2-1 2-2 2-3 3 Area h ≦ 1.155 1.155 ≦ h ≦ 1.450 1.450 ≦ h ≦1.87 — Aspheric κ −5.7045E−01 −6.2289E−01 −6.3685E−01 −4.7663E+01surface A0 0.0000E+00 −1.3522E−02 4.3417E−03 0.0000E+00 coefficient A45.6828E−03 9.2482E−03 8.6397E−03 9.0804E−02 A6 −5.9469E−04 −8.9197E−04−3.5075E−04 −9.6905E−02 A8 3.1229E−03 2.1990E−03 2.9495E−03 7.7081E−02A10 −3.6558E−03 −1.2799E−03 −1.5013E−03 −4.3984E−02 A12 1.5246E−032.1205E−04 2.2836E−04 1.4353E−02 A14 6.2448E−04 2.4175E−04 2.3563E−04−2.0115E−03 A16 −3.5972E−04 −1.6387E−04 −1.6702E−04 1.5954E−05 A18−2.5117E−04 4.6512E−05 4.5239E−05 0.0000E+00 A20 1.2493E−04 −5.6202E−06−4.5598E−06 0.0000E+00 Optical Diffraction 1/−1/−2 0/−1/* 4/*/* pathorder m difference B2 −1.1904E+01 −1.7973E+00 −2.4246E+00 function B45.8012E−01 −1.3271E+00 −1.3384E+00 B6 2.1642E−01 1.7843E+00 1.1618E+00B8 −4.5926E−01 −7.5784E−01 −4.0106E−01 B10 1.8665E−01 1.1876E−014.9987E−02 Wavelength characteristic: +5 nm ΔSA3: 0.062 ΔSA5: 0.013 |d(n− 1)/λ₁ * N| = 12

Example 3

Table 3 shows lens data of Example 3. In Example 3, the central-areadiffractive structure has a five-level structure shown in FIG. 6 b. Afirst-order diffracted light flux has a maximum diffraction-light amountamong diffracted light fluxes generated when the first light flux entersthe central-area diffractive structure, a minus-first-order diffractedlight flux has a maximum diffraction-light amount among diffracted lightfluxes generated when the second light flux enters the central-areadiffractive structure, and a minus-second-order diffracted light fluxhas a maximum diffraction-light amount among diffracted light fluxesgenerated when the third light flux enters the central-area diffractivestructure. The intermediate-area diffractive structure has a three-levelstep structure shown in FIG. 6 d. A zeroth-order diffracted light fluxhas a maximum diffraction-light amount among diffracted light fluxesgenerated when the first light flux enters the intermediate-areadiffractive structure, and a minus-first-order diffracted light flux hasa maximum diffraction-light amount among diffracted light fluxesgenerated when the second light flux enters the intermediate-areadiffractive structure. The peripheral-area diffractive structure may hasany one of a blaze structure and a step structure, and is a structurethat a fourth-order diffracted light flux has a maximumdiffraction-light amount among diffracted light fluxes generated whenthe first light flux enters the peripheral-area diffractive structure.

In Example 3, ΔSA3 is 0.054 λrms and ΔSA5 is 0.009 λrms, which satisfythe above described expression (1), and ΔSA3 ΔSA5=6:1 holds. Thematerial of the objective optical element is plastic. The value of|N*d*(n−1)/λ₁| is 16 and it satisfies the above described expression(5).

Further, the present example satisfies the conditional expression (2)defined by ΔW=C_(SAL)(20ρ⁶+6βρ⁴−6(3+β)ρ²+(4+β))+SAH. The value ofC_(SAL) is 0.009 when β=1, and the value of SAH is 0.016 λrms, whichsatisfy the above expressions (3) and (4).

TABLE 3 (Example 3) Focal length of the f₁ = 2.20 mm f₂ = 2.35 mm f₃ =2.45 mm objective lens Numerical aperture NA1: 0.85 NA2: 0.61 NA3: 0.46Magnification m1: 0 m2: −1/63 m3: −1/89 i-th di ni di ni di ni surfaceri (405 nm) (405 nm) (658 nm) (658 nm) (783 nm) (783 nm) 0 ∞ 150.0 220.01 0.0 0.0 0.0 (Stop (φ3.74 mm) (φ2.87 mm) (φ2.30 mm) diameter) 2-11.4848 2.680 1.5592 2.680 1.5397 2.680 1.5363 2-2 1.4633 2-3 1.5024 3−2.7532 0.685 0.585 0.395 4 ∞ 0.0875 1.6196 0.600 1.5773 1.100 1.5709 5∞ Surface No. 2-1 2-2 2-3 3 Area h ≦ 1.155 1.155 ≦ h ≦ 1.450 1.450 ≦ h ≦1.87 — Aspheric κ −5.7122E−01 −6.2310E−01 −6.3676E−01 −4.7296E+01surface A0 0.0000E+00 −1.3568E−02 4.1590E−03 0.0000E+00 coefficient A45.5923E−03 9.2430E−03 8.8214E−03 9.1402E−02 A6 −3.2321E−04 −9.1128E−04−3.8453E−04 −9.7518E−02 A8 2.7872E−03 2.1917E−03 2.9252E−03 7.6829E−02A10 −3.4535E−03 −1.2786E−03 −1.5061E−03 −4.3926E−02 A12 1.4793E−032.1422E−04 2.2844E−04 1.4449E−02 A14 6.2452E−04 2.4350E−04 2.3595E−04−2.0459E−03 A16 −3.5971E−04 −1.6359E−04 −1.6694E−04 1.6308E−05 A18−2.5117E−04 4.5302E−05 4.5225E−05 0.0000E+00 A20 1.2493E−04 −5.3221E−06−4.5863E−06 0.0000E+00 Optical Diffraction 1/−1/−2 0/−1/* 4/*/* pathorder m difference B2 −1.1951E+01 −1.8039E+01 −2.3803E+00 function B48.0748E−01 −1.2487E+00 −1.3403E−+00 B6 −2.0631E−01 1.7765E+00 1.1645E+00B8 −1.2169E−01 −7.7367E−01 −4.0263E−01 B10 9.0048E−02 1.2303E−014.8046E−02 Wavelength characteristic: +5 nm ΔSA3: 0.054 ΔSA5: 0.009 SAH:0.016 C_(SAL): 0.009 |d(n − 1)/λ₁ * N| = 16 α: 1

Example 4

Tables 4 and 5 show lens data of Example 4. In Example 4, thecentral-area diffractive structure is formed by a diffractive structureof a two-level step type shown in FIG. 6 c and a blaze diffractivestructure which are overlapped together. A zeroth-order diffracted lightflux has a maximum diffraction-light amount among diffracted lightfluxes generated when the first light flux enters the two-level-stepdiffractive structure, a zeroth-order diffracted light flux has amaximum diffraction-light amount among diffracted light fluxes generatedwhen the second light flux enters the two-level-step diffractivestructure, and a ±first-order diffracted light flux has a maximumdiffraction-light amount among diffracted light fluxes generated whenthe third light flux enters the second-level-step diffractive structure.A second-order diffracted light flux has a maximum diffraction-lightamount among diffracted light fluxes generated when the first light fluxenters the blaze diffractive structure, a first-order diffracted lightflux has a maximum diffraction-light amount among diffracted lightfluxes generated when the second light flux enters the blaze diffractivestructure, and a first-order diffracted light flux has a maximumdiffraction-light amount among diffracted light fluxes generated whenthe third light flux enters the blaze diffractive structure. Theintermediate-area diffractive structure has a blaze diffractivestructure. A second-order diffracted light flux has a maximumdiffraction-light amount among diffracted light fluxes generated whenthe first light flux enters the intermediate-area diffractive structure,and a first-order diffracted light flux has a maximum diffraction-lightamount among diffracted light fluxes generated when the second lightflux enters the intermediate-area diffractive structure. Theperipheral-area diffractive structure has a blaze structure, and asecond-order diffracted light flux has a maximum diffraction-lightamount among diffracted light fluxes generated when the first light fluxenters the peripheral-area diffractive structure.

In Example 4, ΔSA3 is 0.102 λrms and ΔSA5 is 0.012 λrms, which satisfythe above described expression (1), and ΔSA3:ΔSA5=8.5:1 holds. Thematerial of the objective optical element is plastic. The value of|d(n−1)/λ₁*N| is 16 and it satisfies the above described expression (5).

Further, the present example satisfies the conditional expression (2)defined by ΔW=C_(SAL)(20ρ⁶+6βρ⁴−6(3+β)ρ²+(4+β))+SAH. The value ofC_(SAL) is 0.012 when β=3.5, and the value of SAH is 0.025 λrms, whichsatisfy the above expressions (3) and (4).

TABLE 4 (Example 4) Focal length of the f₁ = 2.20 mm f₂ = 2.28 mm f₃ =2.45 mm objective lens Numerical aperture NA1: 0.85 NA2: 0.60 NA3: 0.47Magnification m1: 0 m2: −1/65 m3: −1/63 i-th di ni di ni di ni surfaceri (405 nm) (405 nm) (660 nm) (660 nm) (785 nm) (785 nm) 0 ∞ 150.0 155.01 0.0 0.0 0.0 (Stop (φ3.74 mm) (φ2.87 mm) (φ2.30 mm) diameter) 2-11.5606 2.680 1.5592 2.680 1.5397 2.680 1.5363 2-2 1.5623 2-3 1.5641 2-41.5664 2-5 1.4916 3 −2.7124 0.690 0.476 0.414 4 ∞ 0.0875 1.6196 0.6001.5773 1.100 1.5709 5 ∞

TABLE 5 Surface No. 2-1 2-2 2-3 2-4 2-5 3 Area h ≦ 1.190 1.190 ≦ h ≦1.264 1.264 ≦ h ≦ 1.315 1.315 ≦ h ≦ 1.3725 1.3725 ≦ h ≦ 1.87 Aspheric κ−5.7034E−01 −5.7034E−01 −5.7034E−01 −5.7034E−01 −6.3791E−01 −4.0969E+01surface A0 0.0000E+00 −3.5725E−03 −7.1387E−03 −1.0446E−02 −1.0899E−020.0000E+00 coefficient A4 1.4958E−02 1.4958E−02 1.4958E−02 1.4958E−021.0947E−02 9.4599E−02 A6 2.2061E−03 2.2061E−03 2.2061E−03 2.2061E−03−1.0242E−03 −1.0089E−01 A8 2.1796E−03 2.1796E−03 2.1796E−03 2.1796E−032.7913E−03 7.7343E−02 A10 −1.5590E−03 −1.5590E−03 −1.5590E−03−1.5590E−03 −1.4413E−03 −4.3086E−02 A12 3.8539E−04 3.8539E−04 3.8539E−043.8539E−04 2.5039E−04 1.4794E−02 A14 4.6294E−04 4.6294E−04 4.6294E−044.6294E−04 2.3562E−04 −2.6805E−03 A16 −3.3896E−04 −3.3896E−04−3.3896E−04 −3.3896E−04 −1.6776E−04 1.8470E−04 A18 7.7910E−05 7.7910E−057.7910E−05 7.7910E−05 4.4700E−05 0.0000E+00 A20 −3.8230E−06 −3.8230E−06−3.8230E−06 −3.8230E−06 −4.4740E−06 0.0000E+00 First Diffraction 2/1/12/1/1 2/1/1 2/1/1 2/1/1 optical path order m difference B2 −1.6279E+01−1.6279E+01 −1.6279E+01 −1.6279E+01 −6.6701E−01 function B4 6.2385E+006.2385E+00 6.2385E+00 6.2385E+00 1.1269E−01 B6 1.5918E+00 1.5918E+001.5918E+00 1.5918E+00 7.0819E−02 B8 −4.5760E−01 −4.5760E−01 −4.5760E−01−4.5760E−01 2.1757E−02 B10 1.5384E−01 1.5384E−01 1.5384E−01 1.5384E−014.6060E−03 Second Diffraction 0/0/1 optical path order m difference B23.0327E+01 function B4 −2.7763E+00 B6 1.2914E+00 B8 −3.3242E−01 B105.1159E−02 Wavelength characteristic: +5 nm ΔSA3: 0.102 ΔSA5: 0.012 SAH:0.025 C_(SAL): 0.012 |d(n − 1)/λ₁ * N| = 16 α: 3.5

Example 5

Table 6 shows lens data of Example 5. In Example 5, the central-area diactive structure is a structure formed by two types of blaze diffractivestructures which are overlapped together, where the structure has beendescribed to be formed by the first basic structure and the second basicstructure which are overlapped together. The first basic structure is ablaze diffractive structure wherein step differences face the directionopposite to the optical axis. Diffracted light fluxes whose absolutediffraction order is first has a maximum diffraction-light amount amongdiffracted light fluxes generated when the first light flux enters thefirst basic structure, diffracted light fluxes whose absolutediffraction order is first has a maximum diffraction-light amount amongdiffracted light fluxes generated when the second light flux enters thefirst basic structure, and diffracted light fluxes whose absolutediffraction order is first has a maximum diffraction-light amount amongdiffracted light fluxes generated when the third light flux enters thefirst basic structure. The second basic structure is a blaze diffractivestructure wherein step differences face the direction of the opticalaxis. Diffracted light fluxes whose absolute diffraction order is secondhas a maximum diffraction-light amount among diffracted light fluxesgenerated when the first light flux enters the second basic structure,diffracted light fluxes whose absolute diffraction order is first has amaximum diffraction light amount among diffracted light fluxes generatedwhen the second light flux enters the second basic structure, anddiffracted light fluxes whose absolute diffraction order is first has amaximum diffraction-light amount among diffracted light fluxes generatedwhen the third light flux enters the second basic structure. Theintermediate-area diffractive structure is a structure wherein a blazestructure which is similar as the above first basic structure andanother blaze structure which is similar as the above second basicstructure are overlapped together, and a two-level-step diffractivestructure is further overlapped with them. A zeroth-order diffractedlight flux has a maximum diffraction-light amount among diffracted lightfluxes generated when the first light flux enters the two-level-stepdiffractive structure, a zeroth-order diffracted light flux has amaximum diffraction-light amount among diffracted light fluxes generatedwhen the second light flux enters the two-level-step diffractivestructure, and a ±first-order diffracted light flux has a maximumdiffraction-light amount among diffracted light fluxes generated whenthe third light flux enters the two-level-step diffractive structure.The peripheral-area diffractive structure has a blaze structure, and asecond-order diffracted light flux has a maximum diffraction-lightamount among diffracted light fluxes generated when the first light fluxenters the peripheral-area diffractive structure.

In Example 5, ΔSA3 is 0.105 λms and ΔSA5 is 0.024 λrms, which satisfythe above described expression (1), and ΔSA3:ΔSA5=4.4:1 holds. Thematerial of the objective optical element is plastic. The value of|d(n−1)/λ₁*N| is 45 and it satisfies the above described expression (5).

TABLE 6 (Example 5) Focal length of the f₁ = 2.20 mm f₂ = 2.38 mm f₃ =2.45 mm objective lens Numerical aperture NA1: 0.85 NA2: 0.60 NA3: 0.47Magnification m1: 0 m2: 0 m3: 0 i-th di ni di ni di ni surface ri (405nm) (405 nm) (660 nm) (660 nm) (785 nm) (785 nm) 0 ∞ ∞ ∞ 1 0.0 0.0 0.0(Stop (φ3.74 mm) (φ2.87 mm) (φ2.30 mm) diameter) 2-1 1.3099 2.670 1.54142.670 1.5225 2.670 1.5193 2-2 1.5097 2-3 1.4723 3 −2.3669 0.721 0.6460.359 4 ∞ 0.0875 1.6196 0.600 1.5773 1.200 1.5709 5 ∞ Surface No. 2−12−2 2−3 3 Area h ≦ 1.180 1.180 ≦ h ≦ 1.45 1.45 ≦ h ≦ 1.87 Aspheric κ−8.7226E−01 −3.7413E−01 −5.9930E−01 −3.3091E+01 surface A0 0.0000E+002.4521E−02 2.2268E−02 0.0000E+00 coefficient A4 7.9383E−03 2.3786E−021.9406E−02 1.0060E−01 A6 5.4165E−03 −1.8940E−03 −1.0124E−04 −9.9722E−02A8 3.1408E−04 −3.0942E−04 2.4046E−03 7.7657E−02 A10 −1.3516E−03−2.2047E−03 −1.5974E−03 −4.3120E−02 A12 5.1208E−04 5.9886E−04 2.3273E−041.4491E−02 A14 7.0800E−04 3.2379E−04 2.3920E−04 −2.5798E−03 A16−7.9609E−04 −2.2751E−04 −1.6547E−04 1.8060E−04 A18 3.2163E−04 8.4526E−054.5018E−05 0.0000E+00 A20 −4.3814E−05 −1.5640E−05 −4.6358E−06 0.0000E+00First Diffraction 1/1/1 1/1/1 2/1/1 optical path order m difference B26.3821E+01 6.4208E+01 1.4185E+01 function B4 −6.0360E+00 −6.3479E+001.7360E+00 B6 3.1232E+00 2.4210E+00 −1.7979E−01 B8 −1.3062E+00−5.1565E−01 −1.0132E−01 B10 2.5156E−01 5.8699E−02 −4.6009E−02 SecondDiffraction 2/1/1 2/1/1 — optical path order m difference B2 −7.6263E+00−7.6722E+00 — function B4 −3.7264E+00 −4.1548E+00 — B6 1.5761E+001.4111E+00 — B8 −9.7167E−01 −3.8603E−01 — B10 2.3713E−01 7.2573E−02 —Third Diffraction — 0/0/1 — optical path order m difference B2 —−9.4827E+01 — function B4 — 1.9720E+02 — B6 — −1.5525E+02 — B8 —5.5362E+01 — B10 — −7.4420E+00 — Wavelength characteristic: +5 nm ΔSA3:0.105 ΔSA5: 0.024 |d(n − 1)/λ₁ * N| = 45 α: 4.4

REFERENCE SIGNS LIST

-   -   AC1 Two-axis actuator    -   BS Polarization beam splitter    -   DP Dichroic prism    -   CN Central area    -   COL Collimation lens    -   LD1 Semiconductor laser    -   LD2 Semiconductor laser    -   LD3 Semiconductor laser    -   LDP Laser unit    -   MD Peripheral area    -   OL Objective optical element    -   OT Most peripheral area    -   PD Light-receiving element    -   PL1 Protective substrate    -   PL2 Protective substrate    -   PL3 Protective substrate    -   PU1 Optical pickup device    -   QWP Quarter wavelength plate    -   RL1 Information recording surface    -   RL2 Information recording surface    -   RL3 Information recording surface    -   SCN Central spot portion    -   SMD Intermediate spot portion    -   SEN Sensor lens    -   SOT Peripheral spot portion

1. An objective optical element for use in an optical pickup devicecomprising a first light source emitting a first light flux with awavelength λ₁ (375 nm≦λ₁≦435 nm), a second light flux emitting a secondlight flux with a wavelength λ₂ (λ₁<λ₂), and an objective opticalelement, wherein the optical pickup device records and/or producesinformation by converging the first light flux onto an informationrecording surface of a first optical disc including a protective layerwith a thickness t1 and converging the second light flux onto aninformation recording surface of a second optical disc including aprotective layer with a thickness t2 (t1<t2) to record and/or reproduceinformation, using the objective optical element, wherein the objectiveoptical element is a single lens and comprises a central area includingan optical axis and a peripheral area arranged around the central area,wherein a central-area diffractive structure is arranged in the centralarea, the first light flux which has passed through the central area isconverged on the information recording surface of the first optical discso that information can be recorded and/or reproduced, the second lightflux which has passed through the central area is converged on theinformation recording surface of the second optical disc so thatinformation can be recorded and/or reproduced, the first light fluxwhich has passed through the peripheral area is converged on theinformation recording surface of the first optical disc so thatinformation can be recorded and/or reproduced, the second light fluxwhich has passed through the peripheral area is not converged on theinformation recording surface of the second optical disc so thatinformation can be recorded and/or reproduced, and the objective opticalelement satisfies the following expressions, where SA3(λ₁₁), SA5(λ₁₁),SA3(λ₁₂), and SA5(λ₁₂) are third-order spherical aberrations in units ofλrms and fifth-order aberrations obtained when light fluxes with twodifferent wavelengths λ₁₁ and λ₁₂ which are within the range of thewavelength λ₁ and satisfy λ₁₁<λ₁₂ and λ₁₂−λ₁₁=5 nm enter the objectiveoptical element and wavefront aberrations are measured:0.18>ΔSA3>ΔSA5>0  (1) wherein ΔSA3=|SA3(λ₁₂)−SA3(λ₁₁)| andΔSA5=|SA5(λ₁₂)−SA5(λ₁₁)|.
 2. The objective optical element of claim 1satisfying the following expression:0.13>ΔSA3>0.03>ΔSA5>0  (1′).
 3. The objective optical element of claim 1satisfying the following expression:0.18>ΔSA3>0.06>ΔSA5>0  (1″).
 4. The objective optical element of claim 1satisfying ΔSA3:ΔSA5=α:1, wherein the value of α satisfies 4≦α≦9.
 5. Theobjective optical element of claim 1, wherein the objective opticalelement is used for the optical pickup device further comprising a thirdlight source emitting a third light flux with a wavelength λ₃ (λ₂<λ₃),wherein the optical pickup device records and/or produces information byconverging the third light flux onto an information recording surface ofa third optical disc including a protective layer with a thickness t3(t2<t3) to record and/or reproduce information, using the objectiveoptical element, wherein the objective optical element further comprisesan intermediate area arranged between the central area and theperipheral area, the first light flux which has passed through thecentral area is converged on the information recording surface of thefirst optical disc so that information can be recorded and/orreproduced, the second light flux which has passed through the centralarea is converged on the information recording surface of the secondoptical disc so that information can be recorded and/or reproduced, thethird light flux which has passed through the central area is convergedon the information recording surface of the third optical disc so thatinformation can be recorded and/or reproduced, the first light fluxwhich has passed through the intermediate area is converged on theinformation recording surface of the first optical disc so thatinformation can be recorded and/or reproduced, the second light fluxwhich has passed through the intermediate area is converged on theinformation recording surface of the second optical disc so thatinformation can be recorded and/or reproduced, the third light fluxwhich has passed through the intermediate area is not converged on theinformation recording surface of the third optical disc so thatinformation can be recorded and/or reproduced, the first light fluxwhich has passed through the peripheral area is converged on theinformation recording surface of the first optical disc so thatinformation can be recorded and/or reproduced, the second light fluxwhich has passed through the peripheral area is not converged on theinformation recording surface of the second optical disc so thatinformation can be recorded and/or reproduced, and the third light fluxwhich has passed through the peripheral area is not converged on theinformation recording surface of the third optical disc so thatinformation can be recorded and/or reproduced.
 6. The objective opticalelement claim 1 satisfying the following expressions, where W(λ₁₁) andW(λ₁₂) are wavefront aberrations obtained when light fluxes with thewavelength λ₁₁ and the wavelength λ₁₂ (λ₁₁<λ₁₂) enter the objectiveoptical element and wavefront aberrations are measured:ΔW=W(λ₁₂)−W(λ₁₁)ΔW=C _(SAL)(20ρ⁶+6βρ⁴−6(3+β)ρ²+(4+β))+SAH  (2), where W is a wavefrontaberration at a best focus in units of λrms, ρ is a relative pupildiameter under an assumption that a value at a center of an effectivediameter is 0 and a value at a height of an outermost position is 1,C_(SAL) is a coefficient of low-order spherical aberrations, SAH isspherical aberrations with seventh and more orders in units of λrms, andβ is an arbitral value within a range of 0≦β≦4.
 7. The objective opticalelement of claim 6, satisfying the following expression:−0.030≦SAH≦0.030  (3).
 8. The objective optical element of claim 6,satisfying the following expression:0.00<C _(SAL)<0.03  (4).
 9. The objective optical element of claim 1wherein a diffracted light flux with a diffraction order other than azero-th order has a maximum diffracted-light amount among diffractedlight fluxes generated when the first light flux enters the central-areadiffractive structure.
 10. The objective optical element of claim 1satisfying the following expression:0≦|N*d(n−1)/λ₁|≦50  (5), where d is an average of step differences ofring-shaped zones [nm] of a diffractive structure arranged in theperipheral area, n is a refractive index of a material of the objectiveoptical element at the wavelength λ₁, and N is a number of thering-shaped zones of the diffractive structure arranged in theperipheral area.
 11. The objective optical element of claim 10, whereinthe peripheral area is a refractive surface, in the objective opticalelement.
 12. The objective optical element of claim 10, wherein theperipheral area comprises a diffractive structure, in the objectiveoptical element.
 13. An optical pickup device comprising: a first lightsource emitting a first light flux with a wavelength λ₁ (375 nm≦λ₁≦435mm); a second light flux emitting a second light flux with a wavelengthλ₂ (λ₁<λ₂), and an objective optical element, wherein the optical pickupdevice records and/or produces information by converging the first lightflux onto an information recording surface of a first optical discincluding a protective layer with a thickness t1 and converging thesecond light flux onto an information recording surface of a secondoptical disc including a protective layer with a thickness t2 (t1<t2) torecord and/or reproduce information, using the objective opticalelement, wherein the objective optical element is a single lens andcomprises a central area including an optical axis and a peripheral areaarranged around the central area, wherein a central-area diffractivestructure is arranged in the central area, the first light flux whichhas passed through the central area is converged on the informationrecording surface of the first optical disc so that information can berecorded and/or reproduced, the second light flux which has passedthrough the central area is converged on the information recordingsurface of the second optical disc so that information can be recordedand/or reproduced, the first light flux which has passed through theperipheral area is converged on the information recording surface of thefirst optical disc so that information can be recorded and/orreproduced, the second light flux which has passed through theperipheral area is not converged on the information recording surface ofthe second optical disc so that information can be recorded and/orreproduced, and the objective optical element satisfies the followingexpressions, where SA3(λ₁₁), SA5(λ₁₁), SA3(λ₁₂), and SA5(λ₁₂) arethird-order spherical aberrations in units of λrms and fifth-orderaberrations obtained when light fluxes with two different wavelengthsλ₁₁ and λ₁₂ which are within the range of the wavelength λ₁ and satisfyλ₁₁<λ₁₂ and λ₁₂−λ₁₁=5 nm enter the objective optical element andwavefront aberrations are measured:0.18>ΔSA3>ΔSA5>0  (1) wherein ΔSA3=|SA3(λ₁₂)−SA3(λ₁₁)| andΔSA5=|SA5(λ₁₂)−SA5(λ₁₁)|.
 14. The optical pickup device of claim 13,further comprising a third light source emitting a third light flux witha wavelength λ₃ (λ₂<λ₃), wherein the optical pickup device recordsand/or produces information by converging the third light flux onto aninformation recording surface of a third optical disc including aprotective layer with a thickness t3 (t2<t3) to record and/or reproduceinformation, using the objective optical element, wherein the objectiveoptical element further comprises an intermediate area arranged betweenthe central area and the peripheral area, the first light flux which haspassed through the central area is converged on the informationrecording surface of the first optical disc so that information can berecorded and/or reproduced, the second light flux which has passedthrough the central area is converged on the information recordingsurface of the second optical disc so that information can be recordedand/or reproduced, the third light flux which has passed through thecentral area is converged on the information recording surface of thethird optical disc so that information can be recorded and/orreproduced, the first light flux which has passed through theintermediate area is converged on the information recording surface ofthe first optical disc so that information can be recorded and/orreproduced, the second light flux which has passed through theintermediate area is converged on the information recording surface ofthe second optical disc so that information can be recorded and/orreproduced, the third light flux which has passed through theintermediate area is not converged on the information recording surfaceof the third optical disc so that information can be recorded and/orreproduced, the first light flux which has passed through the peripheralarea is converged on the information recording surface of the firstoptical disc so that information can be recorded and/or reproduced, thesecond light flux which has passed through the peripheral area is notconverged on the info nation recording surface of the second opticaldisc so that information can be recorded and/or reproduced, and thethird light flux which has passed through the peripheral area is notconverged on the information recording surface of the third optical discso that information can be recorded and/or reproduced.
 15. The opticalpickup device of claim 13, further comprising a magnification changingmeans arranged at a position between the first light source and theobjective optical element.